Feat: Pending Messages

[Zetanova]

The protocol need to have some ambient information of the current state of the message stream/buffer.

One of the most useful information to give to a message-handler is the pending-message-count
or at least if a flag for message-pending yes/no/unknown

This will help a consumer to decide when to flush/commit a batch/process,
without it, it is very hard to detect idle periods in a message stream.