Add additional debug logging to MultiWriterIdGenerator.__init__ and get_cache_dict

We should add debug logging to MultiWriterIdGenerator similar to that in StreamIdGenerator, per https://github.com/matrix-org/synapse/issues/9268#issuecomment-770190320:

If MultiWriterIdGenerator.__init__ and get_cache_dict included some logger.debug, this would be a lot less confusing to diagonose.

(I mention MultiWriterIdGenerator specifically, because StreamIdGenerator.__init__ does write debug, via _load_current_id, and it is quite helpful, and the inconsistency is extra confusing.)