Utility additions and fixes

1. IPSubnetUtil changed to value class, and String -> Protobuf conversion added.
2. UUIDUtil changed to value class, and nextUuid operation added.
3. StringUtil added with indent method.

Change-Id: Ieb5e2cf71eec6f7f7ffc4aa747099944182e0499
Signed-off-by: Brandon Berg <bberg@midokura.com>
Merge "Utility additions and fixes"