runtime: pace the scavenger according to 1% of overall CPU time

Currently the scavenger is paced to 1% of 1 CPU because it had
scalability problems. As of the last few CLs, that should be largely
resolved. This change resolves the TODO and paces the scavenger
according to 1% of overall CPU time.

This change is made separately to allow it to be more easily rolled
back.

Change-Id: I1ab4de24ba41c564960701634a128a813c55ece9
Reviewed-on: https://go-review.googlesource.com/c/go/+/358675


Trust: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>