MB-11629: In CompactVBTask, set completeBeforeShutdown to true
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 <333a3f71d3ad6e01fc953c4fa06bc1f3cd014910@couchbase.com>
Tested-by: abhinav dangeti <d73ef2279de6577f7bf081737ee47554bbb7dc82@couchbase.com>

couchbase/ep-engine,teligent-ru/ep-engine,owendCB/ep-engine,teligent-ru/ep-engine,teligent-ru/ep-engine,owendCB/ep-engine,hisundar/ep-engine,daverigby/kv_engine,jimwwalker/ep-engine,couchbase/ep-engine,daverigby/kv_engine,sriganes/ep-engine,sriganes/ep-engine,membase/ep-engine,daverigby/kv_engine,membase/ep-engine,sriganes/ep-engine,hisundar/ep-engine,hisundar/ep-engine,daverigby/ep-engine,jimwwalker/ep-engine,owendCB/ep-engine,daverigby/ep-engine,daverigby/ep-engine,membase/ep-engine,daverigby/ep-engine,jimwwalker/ep-engine,couchbase/ep-engine,daverigby/kv_engine,jimwwalker/ep-engine,hisundar/ep-engine,membase/ep-engine,teligent-ru/ep-engine,sriganes/ep-engine,couchbase/ep-engine,owendCB/ep-engine