perf: use application:get_env/3 instead of application:get_env/1

[SergeTupchiy]

application:get_env/1 calls ets:match/2 on ac_tab and can be very inefficient.

[SergeTupchiy]

Some eprof stats from emqx_resource_buffer_worker before this fix:

****** Process <0.5384.0>    -- 0.11 % of profiled time *** 
FUNCTION                                                                                    CALLS        %     TIME  [uS / CALLS]
--------                                                                                    -----  -------     ----  [----------]
...
emqx_placeholder:proc_tmpl/3                                                                12564     0.44     5990  [      0.48]
replayq:pop/5                                                                               11530     0.44     5997  [      0.52]
erlang:monitor/2                                                                             6282     0.45     6049  [      0.96]
mc_worker_api:assign_id/1                                                                    3141     0.45     6068  [      1.93]
erlang:external_size/1                                                                       5765     0.46     6148  [      1.07]
lists:all/2                                                                                  3141     0.47     6278  [      2.00]
emqx_bridge_mongodb_connector:on_query/3                                                     3141     0.47     6366  [      2.03]
mongo_id_server:object_id/0                                                                  3141     0.49     6633  [      2.11]
emqx_resource_buffer_worker:aggregate_counters/2                                             8957     0.53     7156  [      0.80]
erlang:list_to_binary/1                                                                      6282     0.57     7726  [      1.23]
erts_internal:map_next/3                                                                    12132     0.58     7769  [      0.64]
maps:try_next/2                                                                             52880     0.62     8342  [      0.16]
maps:update_with/4                                                                           8906     0.63     8450  [      0.95]
gproc:get_value/2                                                                            5765     0.64     8601  [      1.49]
emqx_utils_json:safe_decode/2                                                               15705     0.66     8921  [      0.57]
emqx_resource_buffer_worker:do_flush/2                                                       3141     0.66     8953  [      2.85]
emqx_bridge_mongodb_connector:'-create_mapping_of_references_to_tuple_values/1-fun-0-'/3    43974     0.69     9276  [      0.21]
emqx_placeholder:'-proc_tmpl/3-fun-0-'/3                                                    37692     0.69     9282  [      0.25]
emqx_placeholder:lookup_var/2                                                               31410     0.72     9747  [      0.31]
emqx_metrics_worker:get_pterm/1                                                              6380     0.79    10623  [      1.67]
erlang:binary_to_existing_atom/2                                                            15705     0.80    10783  [      0.69]
erlang:send/2                                                                                8905     0.87    11698  [      1.31]
erlang:spawn/3                                                                               3141     0.89    11994  [      3.82]
erlang:send/3                                                                                6282     0.90    12079  [      1.92]
gproc_pool:pick_near/3                                                                       5765     1.00    13532  [      2.35]
emqx_resource_buffer_worker:extract_connector_id/1                                           6282     1.11    15020  [      2.39]
emqx_resource_buffer_worker:flush/1                                                          5815     1.17    15768  [      2.71]
lists:map_1/2                                                                               46598     1.20    16191  [      0.35]
rand:seed_put/1                                                                              5766     1.25    16791  [      2.91]
ets:next/2                                                                                   5765     1.39    18781  [      3.26]
emqx_resource_buffer_worker:cancel_flush_timer/1                                             5815     1.44    19476  [      3.35]
erlang:demonitor/2                                                                          12564     1.45    19545  [      1.56]
binary:split/3                                                                               6282     1.52    20493  [      3.26]
maps:fold_1/4                                                                               64978     1.56    20996  [      0.32]
jiffy:nif_decode_init/2                                                                      3141     2.46    33207  [     10.57]
ets:lookup/2                                                                                20624     2.49    33548  [      1.63]
gproc:lookup_value/1                                                                         3141     2.53    34097  [     10.86]
jiffy:decode/2                                                                              18846     3.00    40396  [      2.14]
emqx_placeholder:lookup/2                                                                   15705     3.31    44649  [      2.84]
mc_topology:get_pool/3                                                                       3141     3.32    44742  [     14.24]
emqx_placeholder:do_one_lookup/2                                                            31410     3.60    48480  [      1.54]
ets:lookup_element/3                                                                        12097     3.65    49196  [      4.07]
gen:do_call/4                                                                               12564     7.45   100349  [      7.99]
ets:match/2                                                                                  3141     9.75   131384  [     41.83]
----------------------------------------------------------------------------------------  -------  -------  -------  [----------]
Total:                                                                                    1430744  100.00%  1347828  [      0.94]

[SergeTupchiy]

~40K/s pubs, 4min duration test on 1 EMQX 5.5.0 otp26 node and single mongodb instance (both are c5.4xlarge):

~40K/s pubs, 5min duration test on 1 EMQX 5.5.0 otp26 node and single mongodb instance (both are c5.4xlarge) patched with this fix:

[thalesmg]

🥇