common/mutation.go (KeyVersions, VbConnectionMap)

- Mutation message can carry either KeyVersions or, Vbucket list that is bound
  to the connection.
- KeyVersions for different vbucket (but bounded to same connection) will be
  grouped together as payload for a single Mutation message.
- Added DropData command.

Change-Id: I9ec731bd3207a5da7b0f783e7d45100325f87f6f
Reviewed-on: http://review.couchbase.org/35603
Reviewed-by: Pratap Chakravarthy <prataprc@couchbase.com>
Tested-by: Pratap Chakravarthy <prataprc@couchbase.com>