Make coverage pluggable

[garthk]

If we could replace coverage.js with an API-compatible module at runtime (cf: custom reporters and transform modules), we could experiment with native v8 coverage without an extra transpile step. In turn, that could plausibly fix #856 as a side-effect.

[geek]

@garthk I do want to add support to run coverage through the new v8 coverage, this should also make it easier to track coverage in child processes

[hueniverse]

I would love to use v8 native coverage. This will happen after we stop supporting node 8 in a few months. I do not want to make coverage pluggable as you can already do that by simply not using lab for coverage...

[lock]

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