RPC responses should have the INTERNAL flag cleared

When an RPC request is retransmitted, it will be sent by NAKACK2 or UNICAST3 as INTERNAL message. If the RPC call returns a response, that response will also be INTERNAL, using up the internal thread pool.
SOLUTION: clear the INTERNAL flag on an RPC response. JGroups uses the INTERNAL flag only with messages, never with RPCs.