When scavenging small amounts it's possible we over-scavenge by a
significant margin since we choose to scavenge the largest spans first.
This over-scavenging is never accounted for.

With this change, we add a scavenge credit pool, similar to the reclaim
credit pool. Any time scavenging triggered by RSS growth starts up, it
checks if it can cash in some credit first. If after using all the
credit it still needs to scavenge, then any extra it does it adds back
into the credit pool.

This change mitigates the performance impact of golang.org/cl/159500 on
the Garbage benchmark. On Go1 it suggests some improvements, but most of
that is within the realm of noise (Revcomp seems very sensitive to
GC-related changes, both postively and negatively).

Garbage: https://perf.golang.org/search?q=upload:20190131.5
Go1:     https://perf.golang.org/search?q=upload:20190131.4

Performance change with both changes:

Garbage: https://perf.golang.org/search?q=upload:20190131.7
Go1:     https://perf.golang.org/search?q=upload:20190131.6

Change-Id: I87bd3c183e71656fdafef94714194b9fdbb77aa2
Reviewed-on: https://go-review.googlesource.com/c/160297Reviewed-by: Austin Clements <austin@google.com>