loggers: Set propagate False for zulip.slow_queries logger.
loggers: Set propagate False for zulip.slow_queries logger.

This will prevent it to propagating to root logger and causing flaky
behavior in tests which verify logs by root logger using assertLogs.

rht/zulip,hackerkid/zulip,brainwane/zulip,hackerkid/zulip,rht/zulip,andersk/zulip,rht/zulip,eeshangarg/zulip,zulip/zulip,andersk/zulip,punchagan/zulip,zulip/zulip,kou/zulip,eeshangarg/zulip,rht/zulip,andersk/zulip,eeshangarg/zulip,showell/zulip,brainwane/zulip,rht/zulip,hackerkid/zulip,kou/zulip,kou/zulip,showell/zulip,zulip/zulip,andersk/zulip,hackerkid/zulip,brainwane/zulip,showell/zulip,brainwane/zulip,showell/zulip,zulip/zulip,rht/zulip,rht/zulip,showell/zulip,punchagan/zulip,andersk/zulip,andersk/zulip,brainwane/zulip,andersk/zulip,punchagan/zulip,zulip/zulip,eeshangarg/zulip,punchagan/zulip,kou/zulip,kou/zulip,eeshangarg/zulip,eeshangarg/zulip,brainwane/zulip,hackerkid/zulip,zulip/zulip,punchagan/zulip,hackerkid/zulip,kou/zulip,showell/zulip,zulip/zulip,eeshangarg/zulip,showell/zulip,kou/zulip,punchagan/zulip,punchagan/zulip,hackerkid/zulip,brainwane/zulip