This changeset updates `nomad/volumewatcher` to take advantage of the
`CSIVolume.Unpublish` RPC. This lets us eliminate a bunch of code and
associated tests. The raft batching code can be safely dropped, as the
characteristic times of the CSI RPCs are on the order of seconds or even
minutes, so batching up raft RPCs added complexity without any real world
performance wins.
Includes refactor w/ test cleanup and dead code elimination in volumewatcher
This changeset adds a subsystem to run on the leader, similar to the
deployment watcher or node drainer. The `Watcher` performs a blocking
query on updates to the `CSIVolumes` table and triggers reaping of
volume claims.
This will avoid tying up scheduling workers by immediately sending
volume claim workloads into their own loop, rather than blocking the
scheduling workers in the core GC job doing things like talking to CSI
controllers
The volume watcher is enabled on leader step-up and disabled on leader
step-down.
The volume claim GC mechanism now makes an empty claim RPC for the
volume to trigger an index bump. That in turn unblocks the blocking
query in the volume watcher so it can assess which claims can be
released for a volume.
