runtime: simplify mark termination and eliminate mark 2

[aclements]

The garbage collector currently uses a racy and complex algorithm to enter mark termination, which has wide-ranging consequences for the complexity of the mark termination implementation. I propose replacing it with a race-free distributed termination detection-like algorithm. This should open the door to several significant simplifications of the garbage collector algorithm.

The design doc can be viewed here.

/cc @RLH

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Change https://golang.org/cl/128896 mentions this issue: design: simplify mark termination and eliminate mark 2

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Change https://golang.org/cl/134319 mentions this issue: runtime: eliminate gcBlackenPromptly mode

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Change https://golang.org/cl/134317 mentions this issue: runtime: track whether any buffer has been flushed from gcWork

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Change https://golang.org/cl/134316 mentions this issue: runtime: remove GODEBUG=gctrace=2 mode

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Change https://golang.org/cl/134320 mentions this issue: runtime: don't disable GC work caching during mark termination

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Change https://golang.org/cl/134315 mentions this issue: runtime: flush write barrier buffer to create work

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Change https://golang.org/cl/134318 mentions this issue: runtime: eliminate mark 2 and fix mark termination race

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Change https://golang.org/cl/134778 mentions this issue: runtime: add a more stable isSystemGoroutine mode

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Change https://golang.org/cl/134784 mentions this issue: runtime: eliminate work.markrootdone and second root marking pass

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Change https://golang.org/cl/134779 mentions this issue: runtime: support disabling goroutine scheduling by class

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Change https://golang.org/cl/134780 mentions this issue: runtime: implement STW GC in terms of concurrent GC