MB-11629: In CompactVBTask, set completeBeforeShutdown to true

Possible race condition between handleDisconnect and notify from
CompactVBTask, causing the session counter to be
decremented for a compactVB task before it completes execution,
and when it does run later, results in an underflow of the
session counter.

Change-Id: Ie3681f84925df7bf4cf71672b6625aa43b1c6a4d
Reviewed-on: http://review.couchbase.org/39212
Reviewed-by: Sundararaman Sridharan <sundar@couchbase.com>
Tested-by: abhinav dangeti <abhinav@couchbase.com>