timers: handle unrefed intervals in clearTimeout()

[cjihrig]

Checklist

• make -j8 test (UNIX), or vcbuild test nosign (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines

Affected core subsystem(s)

timers

Description of change

This commit clears the _repeat property of all timer objects in clearTimeout(). This prevents intervals passed to clearTimeout() from being rearmed.

Fixes: #9561 (although it does not directly address wrap->object() issue described in #9561 (comment))

R= @bnoordhuis

[Fishrock123]

I don't think this is an exhaustive patch.

unenroll() should cancel any timer callbacks without fail and Timer#close() should always stop the _handle without fail.

[Fishrock123]

See #9561 (comment)