SRV records aren't handled properly

[jcgruenhage]

Description

As described here in the code, load balancing and failover using SRV records doesn't work properly in Synapse.

[jcgruenhage]

That comment mentions that load balancing is broken and that synapse fails over to lower priority servers without the higher priority servers actually being down. Testing with tulir seemed to indicate that failover when a higher priority server is actually down doesn't work either, but I haven't found an explanation for that in the code and I haven't spent more time trying to reproduce it. Hopefully that will just go away with this piece of the code being reworked.

[jcgruenhage]

Update: Load balancing works now, failover doesn't yet.

[richvdh]

I think this was fixed in #5864