Synapse accepts SRV records that point to a CNAME

Rumour has it that Synapse will follow a `_matrix._tcp` SRV record that points to a CNAME. This is invalid according to RFC2782.

This is a problem, because it means that people will be under the misapprehension that their server is is configured fine, while it won't actually work with standards-respecting homeserver impls.

(Looks like we fixed this in the federation-tester in 2019: https://github.com/matrix-org/matrix-federation-tester/issues/43)