Add ProtobufServer tests. Free receive buffer and protobuf message
in HandleReceive in ProtobufServer. Protect UdpServer pending
buffer array via mutex.

Change-Id: Ibcb7de7a06caa9746d32d66172091641599304bb
Merge "Add ProtobufServer tests. Free receive buffer and protobuf message in HandleReceive in ProtobufServer. Protect UdpServer pending buffer array via mutex."