Encode and Decode mutation messages.

removed UPR specific command names.
renamed command name `Mutation` to `Upsert`.
renamed command name `UprDeletion` to `Deletion`.

Change-Id: I3ef77db708c0b126f0fd94512cff0f9770b11490
Reviewed-on: http://review.couchbase.org/35192
Reviewed-by: Pratap Chakravarthy <prataprc@couchbase.com>
Tested-by: Pratap Chakravarthy <prataprc@couchbase.com>