Merge #990

990: Small pubsub improvements r=Dirbaio a=diondokter

- Futures in pub & sub are now awaited instead of returned
- Added functions for reading how many messages are available

This helps people get better compiler diagnostics. For example, I forgot to call await on a future and the compiler didn't complain.
This also helps with making some decisions based on the state of the channels.

Co-authored-by: Dion Dokter <dion@tweedegolf.com>