The map which defined fieldname -> Analyzer needs to be fully known at IndexWriter initialization time.

We could try to support an "incremental definition" of new field names in such a map, at cost of flushing and reopening the IndexWriter, but it would be much simpler to know the full map in advance.

As a consequence, the analyzer map no longer needs to be part of a transaction payload when sent to the backend. If we decide to still allow an incremental definition rather than needing full advance knowledge, we'll need a new backend command to transmit the new configuration.