Improve RPC framework

• Add a client::register_event command to let the server known that a client needs to be notified.
• Fix hardcoded process ranks
• Add a Callback policy for events so that we now have callback_policy, buffered_policy, unbuffered_policy. Callback policy does not use notification channel and should be used internally only.
• Add support for server with multiple clients
• [Comestic] Rename service in message because it is metafunction that generates message types for commands and events
• [Comestic] Typos or file reorganization