[Feature] Fallback/backup node

[pkesso]

I use chproxy for SELECT distribution. I have a single-node 'cluster' in chproxy config for this, however, when the designated node is not ok, I would like to pass requests to another node, which should not be used, when primary is available.

I woud like to have a 'backup' of 'fallback' node in cluster settings, similar to backup upstream in NGINX - the node to send traffic if primary is not available.

Node weight would be fine too.