Add missing ZMQ notifications

[MorningLightMountain713]

bitcoin/bitcoin#13570

(Almost) All fluxnodes now run fluxd with zmq notifications enabled. (FluxOS isn't using this right now, but it will be for block notifications in the medium term future).

Any nodes that don't use zelcash.service will not have it enabled. As of version 5.25.0, this was < 0.5% of nodes. I will probably look to enable it on the remaining nodes also.

bitcoind has the abillity to query the getzmqnotifications rpc endpoint to see the status of zmq.

It is quite tricky with current fluxd setup as we have to physically check if zmq is reachable which is messy.

This would be quite a good first issue for me to implement - I could look at doing this after I finish current works.