protobuf: format protobuf timestamps for logging.

Change-Id: Ifa7ce45749d4840fd589580ee2d1c4ea1518d516
Reviewed-on: http://review.couchbase.org/44166
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>
Reviewed-by: Pratap Chakravarthy <prataprc@couchbase.com>
Tested-by: Pratap Chakravarthy <prataprc@couchbase.com>