Support services

[dantleech]

Add support for services (e.g. an indexer)

• Be able to start and stop services
• Be able to start services on server initialized call
• Send "notifications" to the client.

Refactoring:

• Refactor to extract a "message publsher" class
• Make the message publisher available in the ApplicationHandlerLoader (e.g. in the connection specific Phpactor instance)
• Services are long running co-routines
• A service manager could start / stop services potentially.