MB-7685: Refactor VBucketDeletion task

Change-Id: I021ca96c0e2e914ea498e4d2d78b9477ac3b5b79
Reviewed-on: http://review.couchbase.org/24870
Reviewed-by: Jin Lim <jin@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>