crtime: add MonoFromTime method

[RaduBerinde]

This is useful when we have a time.Time that we can't change to use
Mono, for example ctx.Deadline().

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This change is 

[nvanbenschoten]

Reviewed 2 of 2 files at r1, all commit messages.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @RaduBerinde)

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crtime/monotonic_test.go line 30 at r1 (raw file):

c > b+Mono(time.Millisecond)

Isn't this part susceptible to flaking due to overload or a poorly timed goroutine/process preemption?

[RaduBerinde]

Reviewable status: 1 of 2 files reviewed, 1 unresolved discussion (waiting on @nvanbenschoten)

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crtime/monotonic_test.go line 30 at r1 (raw file):

Previously, nvanbenschoten (Nathan VanBenschoten) wrote…

c > b+Mono(time.Millisecond)

Isn't this part susceptible to flaking due to overload or a poorly timed goroutine/process preemption?

Possibly, updated.

[nvanbenschoten]

Reviewed 1 of 1 files at r2, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @RaduBerinde)

[RaduBerinde]

TFTR!