MB-100: kvshard2 (May-06)

- MRW12
- IOManager with Kvshard
- Singleton CouchNotifier
- Fixed snapshotvbuckets logic
- Fix bgfetch perf regression
- Fix cancel-shutdown race condition

Change-Id: I6054a4ed579b48338900e87a04e07a136691d37e
Reviewed-on: http://review.couchbase.org/26134
Reviewed-by: abhinav dangeti <abhinav.dangeti@gmail.com>
Tested-by: abhinav dangeti <abhinav.dangeti@gmail.com>