protobuf changes for Snapshot, getvbmap, rebalance.

CBIDXT-140, CBIDXT-99, CBIDXT-130

- added snapshot fields in timestamp messages.
- a helper method to retrieve snapshot marker from vbucket's key-versions.
  * description of interpreting, rather mis-interpreting snapshot-marker from
    a keyversion structure.
- protobuf messages to marshal VBMap from couchbase-client.
  * helper methods on VBMap response.
- Failoverlog messages.
  * helper method to compute BranchTimestamp for `backfill` streams from
    failover-log response.
  * helper method to construct a map of failover logs for each vbucket in
    failover-log response.
- removed panics from helper methods.
- updated RepairDownstreamEndpoints message.

Change-Id: I337aca837d95978d9b07d9f8bbc8f03e4c589284
Reviewed-on: http://review.couchbase.org/40412
Reviewed-by: John Liang <johliang00@gmail.com>
Reviewed-by: Pratap Chakravarthy <prataprc@couchbase.com>
Tested-by: Pratap Chakravarthy <prataprc@couchbase.com>