Following the new volumewatcher in #7794 and performance improvements
to it that landed afterwards, there's no particular reason we should
be threading claim releases through the GC eval rather than writing an
empty `CSIVolumeClaimRequest` with the mode set to
`CSIVolumeClaimRelease`, just as the GC evaluation would do.
Also, by batching up these raft messages, we can reduce the amount of
raft writes by 1 and cross-server RPCs by 1 per volume we release
claims on.
Tim Gross