Since the current revision is 0, it'll always be less than the compaction
revision. If the proxy sees a compaction, it would always reject the
current revision requests since it's less than the compaction revision.
Instead, check if the revision is historical before trying to reject on
compaction revision.
Fixes#7599
groupcache needs a write lock and has no way to expire keys; ccache can
do this, though.
Also removes the key count metric, since there's no way to efficiently
calculate it using ccache.