Fixed a potential deadlock in rpc_message_handler
Fixed a potential deadlock in rpc_message_handler

I'm not sure if this is the cause of our RPC deadlocks, but
rpc_message_handler definitely holds the pending_results_mutex for too
long. It should release it before going into the inner call to
parse_and_receive, because parse_and_receive won't touch the
pending_results_to_fulfill queue and shouldn't be blocking further
rpc sends from the main thread.

Derecho-Project/derecho-unified,Derecho-Project/derecho-unified,Derecho-Project/derecho-unified,Derecho-Project/derecho-unified,Derecho-Project/derecho-unified