GC assists must block until the assist can be satisfied (either
through stealing credit or doing work) or the GC cycle ends.
Currently, this is implemented as a retry loop with a 100 µs delay.
This obviously isn't ideal, as it wastes CPU and delays mutator
execution. It also has the somewhat peculiar downside that sleeping a
G requires allocation, and this requires working around recursive
allocation.

Replace this timed delay with a proper scheduling queue. When an
assist can't be satisfied immediately, it adds the allocating G to a
queue and parks it. Any time background scan credit is flushed, it
consults this queue, directly satisfies the debt of queued assists,
and wakes up satisfied assists before flushing any remaining credit to
the background credit pool.

No effect on the go1 benchmarks. Slightly speeds up the garbage
benchmark.

name              old time/op  new time/op  delta
XBenchGarbage-12  5.81ms ± 1%  5.72ms ± 4%  -1.65%  (p=0.011 n=20+20)

Updates #12041.

Change-Id: I8ee3b6274dd097b12b10a8030796a958a4b0e7b7
Reviewed-on: https://go-review.googlesource.com/15890Reviewed-by: Rick Hudson <rlh@golang.org>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>