Backlog quota to be based on number of messages

**Is your enhancement request related to a problem? Please describe.**
Base Backlog quota on the number of messages, not size or time.

**Describe the solution you'd like**
Have the ability to pass a number_of_messages parameter to set-backlog-quota

**Describe alternatives you've considered**
Using time- or size-based quotas - feels hackish.

See #10268 and #9680.

Start with the set-backlog-quota command and read the related discussions in #10268 and #9680. Define the change around accepting a number_of_messages parameter and verify that backlog quota is based on message count rather than size or time.

Written by the indexing model from the issue text.