Fix cancel-shutdown race condition

Change-Id: I798606f9dd047d684aa6700d0e20713a71b227b2
Kvshard2 - MRW17

- num_shards to 2
- iomanager(2, 2)
- directly scheduled bgfetcher
Merge branch 'kvshard2' of github.com:abhinavdangeti/ep-engine into kvshard2