CBIDXT-87 version numbering for mutation path.

protobuf messages are version number for mutation path. hopefully this will
also address rolling upgrade across upstream and downstream nodes.

Change-Id: I0c6be207af0bafd2d608015083a0dddf97a421f9
Reviewed-on: http://review.couchbase.org/39264
Reviewed-by: Pratap Chakravarthy <prataprc@couchbase.com>
Tested-by: Pratap Chakravarthy <prataprc@couchbase.com>