Extend messaging API with high performance patterns

Currently implemented API of oslo.messaging is incomplete from efficient brokerless usage point of view. In other words it is too oriented on reliable central-broker messaging and ignores more efficient but less reliable multicast etc. The proposal is to provide extended API version (3.0) where new messaging patterns will come into use.

Now we have RPC (CALL, CAST, FANOUT) and Notifications patterns. Lets mark them 'reliable' so their behaviour wouldn't change from client's point of view.

The following new patterns are proposed to observe:

BROAD_CAST (Unreliable Fanout) - Multicast without guarantees of delivery (UDP-like).
PUBLISH - Similar to Notifications, but may drop messages if no subscriber present.
ASYNC_CALL - Reply is returned in Future object, so call wouldn't block the thread.

These patterns wouldn't use acknowledgements which is prerogative of 'reliable' group.