MB-14859: Handle quick successive BG Fetch of a key interleaved with exp pager
MB-14859: Handle quick successive BG Fetch of a key interleaved with exp pager

If two bgfetch are scheduled for a non existing key, and one bgfetch completes
and marks the key as non existant in the hash table, and subsequently expiry
pager removes it from the hash table before the second bgfetch completes,
we need to handle the case appropriately in the complete bgfetch code as
notify the memcached with appropriate return value.

(cherry picked from commit f9402cb0ee6a3592413e43855b0a48b7c0202a5b)

Change-Id: I8eaf54319014ea4039c74d2cbfab21ef275939fe
Reviewed-on: http://review.couchbase.org/64929
Tested-by: buildbot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Manu Dhundi <158873d90a7ef40f3637a222b7329c09d0222554@couchbase.com>
Well-Formed: buildbot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>

teligent-ru/ep-engine,daverigby/kv_engine,teligent-ru/ep-engine,daverigby/kv_engine,couchbase/ep-engine,daverigby/ep-engine,membase/ep-engine,daverigby/kv_engine,daverigby/ep-engine,membase/ep-engine,teligent-ru/ep-engine,couchbase/ep-engine,membase/ep-engine,membase/ep-engine,couchbase/ep-engine,daverigby/ep-engine,couchbase/ep-engine,daverigby/ep-engine,teligent-ru/ep-engine,daverigby/kv_engine