Support for selecting the Redis logical database. (#15034)

Note that this is only used for key-value store (cached values)
and not for the pub/sub replication used by Synapse.

contrib/docker_compose_workers/README.md

@@ -68,6 +68,7 @@ redis:
   enabled: true
   host: redis
   port: 6379
+  # dbid:  <redis_logical_db_id>
   # password: <secret_password>  
 ```