Add helper method for cdb_client to send 'new signal' events

This branch implements function SignalProducer.send_from_cdb_client which allows cdb_client to send a message without knowledge of the message scheme. It partly replaces compass_notifications.shortcuts.send_signal_message which created a new SignalProducer for each message. To avoid such overhead, pyCDBBase class now stores a single instance of SignalProducer and cdb_client uses this new method.