MB-7966 IOManager integration with KVShard

Change-Id: I641dda9ea68bdade2f71b5805da439173dc3d678
Reviewed-on: http://review.couchbase.org/25955
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Reviewed-by: Jin Lim <jin@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>
MB-7729: Fix 100% CPU Issue

The background fetcher jobs todo stat was not being
decremented properly.

Change-Id: I26a35e76074644b63117d348c05123c09691cc44
Reviewed-on: http://review.couchbase.org/26167
Reviewed-by: Jin Lim <jin@couchbase.com>
Tested-by: Jin Lim <jin@couchbase.com>