Create ThreadSafeUuidGenerator to encapsulate acquiring/releasing
mutex when using boost::uuids::random_generator so that clients
do not have to worry about it, as per review comments on
previous checkin.

Change-Id: I7a597e16993057e5450f7c81df06160dc236f99b
Merge "Create ThreadSafeUuidGenerator to encapsulate acquiring/releasing mutex when using boost::uuids::random_generator so that clients do not have to worry about it, as per review comments on previous checkin."