fix: only perform "free" retry upon receiving DOWN signals

[thalesmg]

Addresses #45 (comment)

[thalesmg]

Unfortunately, this leads to the return of the previous errors when there are lots of concurrent requests. Which means some cases are indeed from gun_down messages:

iex(emqx@127.0.0.1)8> 2023-04-27T13:58:24.867919-03:00 [error] msg: http_connector_do_request_failed, mfa: emqx_connector_http:on_query/3, line: 321, connector: <<"bridge:webhook:web">>, reason: normal, request: {"/api/batch_subscribe",[{<<"content-type">>,<<"application/json">>}],<<"{\n  \"clientid\": \"caribdis_bench_sub_1581851150_10829\"\n}">>}
2023-04-27T13:58:24.868215-03:00 [error] msg: unrecoverable_error, mfa: emqx_resource_buffer_worker:handle_query_result_pure/3, line: 823, id: <<"bridge:webhook:web">>, reason: normal