MB-9978 Don't snooze the vbucket visitor task unless requested.

Change-Id: I1d4d7aa57417fb1c1579fe0387b1cf0431b22ec8
Reviewed-on: http://review.couchbase.org/32784
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>