Convert LeaveTracker to singleton GenServer

[coveralls]

Pull Request Test Coverage Report for Build 189

• 2 of 4 (50.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.03%) to 90.278%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
apps/buddy_matching_web/lib/buddy_matching_web/presence/leave_tracker.ex	1	2	50.0%
apps/buddy_matching_web/lib/buddy_matching_web/presence/supervisor.ex	0	1	0.0%
<!--	Total:	2	4	50.0%	-->

Totals
Change from base Build 185:	-0.03%
Covered Lines:	325
Relevant Lines:	360

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💛 - Coveralls

[Hanspagh]

Looks good, out of curiosity is a pattern to use singleton gen servers, or where do you see the benefit?

[ecly]

@Hanspagh Should indeed be a pattern. To avoid having to do :global.whereis_name/1, when wanting to a call a GenServer that we know will be only be created one instance of ever. Makes the interface much easier to work with.

They for example present it in the 'Programming Elixir` book:

As we left it, our server works but is ugly to use. Our callers have to make explicit GenServer calls, and they have to know the registered name for our server process. We can do better... ... This is the pattern you should use in your servers.