Extract some duplicated code for sync triggers and timers

[tgoyne]

A trivial bit of refactoring extracted from one of my multiprocess sync branches. I wrote some new timers and found it silly that I had to copy and paste stuff so I extracted the duplicated code. I also made it so that Trigger accepts any callable plus arguments similar to std::thread.

[coveralls-official]

Pull Request Test Coverage Report for Build thomas.goyne_484

Details

• 62 of 71 (87.32%) changed or added relevant lines in 6 files are covered.
• No unchanged relevant lines lost coverage.
• Overall first build on tg/trigger at 91.097%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/realm/sync/client.cpp	4	6	66.67%
src/realm/sync/trigger.hpp	22	25	88.0%
src/realm/sync/noinst/client_impl_base.cpp	18	22	81.82%

Totals
Change from base Build 2543:	91.1%
Covered Lines:	216695
Relevant Lines:	237872

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💛 - Coveralls