Unique FakeServer when no connection params are provided

[cunla]

Fix #140

[github-actions]

Coverage report:

Name                                              Stmts   Miss Branch BrPart  Cover
-----------------------------------------------------------------------------------
fakeredis/__init__.py                                 7      2      0      0    71%
fakeredis/_basefakesocket.py                        216      7     86      8    95%
fakeredis/_command_args_parsing.py                   71      2     46      2    97%
fakeredis/_commands.py                              295      8    110      5    96%
fakeredis/_fakesocket.py                             19      0      0      0   100%
fakeredis/_helpers.py                               167      4     61      3    97%
fakeredis/_msgs.py                                   64      0      0      0   100%
fakeredis/_server.py                                132      7     40      4    94%
fakeredis/_stream.py                                 92      3     38      5    94%
fakeredis/_zset.py                                   64      0     16      0   100%
fakeredis/aioredis.py                               151      9     41      8    91%
fakeredis/commands_mixins/__init__.py                 0      0      0      0   100%
fakeredis/commands_mixins/bitmap_mixin.py           116      0     58      0   100%
fakeredis/commands_mixins/connection_mixin.py        21      0      4      0   100%
fakeredis/commands_mixins/generic_mixin.py          212      9     90      8    94%
fakeredis/commands_mixins/geo_mixin.py              143     11     63      5    89%
fakeredis/commands_mixins/hash_mixin.py              80      1     12      1    98%
fakeredis/commands_mixins/list_mixin.py             176      5     76      5    96%
fakeredis/commands_mixins/pubsub_mixin.py            81      0     36      6    95%
fakeredis/commands_mixins/scripting_mixin.py        147      3     70      4    97%
fakeredis/commands_mixins/server_mixin.py            47      0     10      0   100%
fakeredis/commands_mixins/set_mixin.py              138     12     58      6    88%
fakeredis/commands_mixins/sortedset_mixin.py        315      7    121      6    97%
fakeredis/commands_mixins/streams_mixin.py           58      3     22      3    92%
fakeredis/commands_mixins/string_mixin.py           217      5    102      6    97%
fakeredis/commands_mixins/transactions_mixin.py      71      2     20      1    97%
fakeredis/geo/__init__.py                             0      0      0      0   100%
fakeredis/geo/geohash.py                             45      0     20      0   100%
fakeredis/geo/haversine.py                           10      0      0      0   100%
fakeredis/stack/__init__.py                           9      5      2      0    36%
fakeredis/stack/_json_mixin.py                      292      9    129      6    96%
-----------------------------------------------------------------------------------
TOTAL                                              3456    114   1331     92    95%