feat: Add optional `keepAlive` option to Meter and Timer

[tuckbick]

This PR unrefs timers by default, eliminating headaches for people unsure about why their unit tests keep hanging.

It also eliminates having to break a 1 line piece of code into 3:

// no unref-ing
collection.meter('responses').mark()

// with unref-ing
const responsesMeter = collection.meter('responses');
responsesMeter.unref();
responsesMeter.mark();

// with PR, keepAlive = false
collection.meter('responses').mark()

// with PR, keepAlive = true
collection.meter('responses', {keepAlive: true}).mark()

I've set the default to false since that was most helpful for my team. However, I could be convinced to have it default to true.

[coveralls]

Pull Request Test Coverage Report for Build 374

• 6 of 6 (100.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.4%) to 89.042%

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Totals
Change from base Build 372:	0.4%
Covered Lines:	759
Relevant Lines:	835

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💛 - Coveralls