MB-11799 re-use existing task for rescheduling vb snapshots

we need not create new task on vb snapshot failure,
re-use the task by returning true from the run method.
Change-Id: I8f3b9988e2c55d95bf23cecad9653c87cdcebe0e
Reviewed-on: http://review.couchbase.org/39958
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Sundararaman Sridharan <sundar@couchbase.com>