MB-20852 [18/N]: Remove now dead VBucket persist Tasks

MB-20852 [18/N]: Remove now dead VBucket persist Tasks

Delete all tasks (apart from Flusher) which used to persist VBucket
state to disk - the Flusher Task now handles all VBucket state
persistence.

Also remove statistics which are no longer valid / relevent -
snapshotVbucketHisto / persistVBStateHisto - we no longer snapshot
separately from flushing.

Change-Id: Ia1b3f30f0f792a8ca739d377443f909138dcaa86
Reviewed-on: http://review.couchbase.org/69151
Reviewed-by: Jim Walker <jim@couchbase.com>
Tested-by: buildbot <build@couchbase.com>