timer: refactor to use module.exports ={} and ES6 classes

[jasnell]

Refactor to use the more efficient module.exports ={} pattern and change up the internal objects to use ES6 classes. Benchmarking shows a minor perf improvement under turbo / ignition (not compelling enough by itself, but it's there).

/cc @Fishrock123

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines

Affected core subsystem(s)

timers

[benjamingr]

Also cc @mscdex

[mscdex]

What are the benchmark results like with Crankshaft (assuming this PR would get backported)?

[Fishrock123]

"good luck backporting this" ... I guess?

[jasnell]

@mscdex ... no significant variance from current master... (although, I must say that master compared to 7.9 is rather concerning independent of this PR).

Backporting shouldn't be a priority at all with this, but the separate module.exports ={} commit should be easily backportable. The ES6 classes one wouldn't need to be.

[TimothyGu]

Is changing to ES6 classes backwards compatible? The prototype methods will no longer be enumerable after such a change.

[jasnell]

@TimothyGu ... good question. It's likely better to be defensive about it tho.