Add new command to wait for channel to be empty

This is useful for drain channels and complete.

The workaround today is to send a special message to tell the consumer about that. This is a bit unnatural.
Also when the number of consumers are more than one, the sender need to publish more than one "special" messages

Start by tracing how the repository represents channels, consumers, and commands, then identify the existing workflow and message-queue entry points involved in draining a channel. Clarify the expected behavior when multiple consumers are present, including how completion is detected and exposed. Done should include the new command and coverage for empty-channel draining with one and multiple consumers.

Written by the indexing model from the issue text.