Adds Light-weight cluster bus header for pubsub message. (#654)

Adds light-weight cluster bus header for pubsub message. Closes #557.

This also supports sending to and receiving non-light messages from
older versions of the engine.

The light-weight cluster bus message supports multiple pubsub messages
(payloads) for one pubsub channel. Receiving messages with multiple
payloads is supported but we're not yet sending such multi-payload
messages to other nodes.

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Signed-off-by: Roshan Khatri <[email protected]>