Avoid doing unneeded logger work in Replication

[tgoyne]

Most of the replication log statements do some work including memory allocations which are then thrown away if the log level it too high, so always check the log level first. A few places don't actually benefit from this, but it's easier to consistently check the log level every time.

This makes the obj-c bulk insert benchmark slightly faster than v13.27.0 rather than ~10% slower.

[coveralls-official]

Pull Request Test Coverage Report for Build thomas.goyne_205

Details

• 63 of 100 (63.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall first build on tg/replication-logging at 90.9%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/realm/replication.cpp	58	95	61.05%

Totals
Change from base Build 2083:	90.9%
Covered Lines:	238311
Relevant Lines:	262169

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💛 - Coveralls

[nirinchev]

Would it make sense to adopt this as a general-purpose approach for logging - maybe wrap it in a macro that would expand to:

if (auto logger = would_log(*log_level*)) {
  logger->log(*log_category*, *log_level*, *message*);
}

Not sure if it would be an optimization across the board, but I can easily imagine other places either now or in the future, where we'd unnecessarily construct messages.

[jedelbo]

@nirinchev I think the issue here is that these activities are the ones most frequently used.

[tgoyne]

The performance impact of unused logging is definitely something we'll need to keep an eye on as we add more of it, but so far this is the only place it's actually been measurable so I think it's still too early to try to build a generalized mitigation.