Fix federation_client to send the right Host
Fix federation_client to send the right Host

This appears to have stopped working since matrix.org moved to cloudflare. The
Host header should match the name of the server, not whatever is in the SRV
record.

matrix-org/synapse,matrix-org/synapse,matrix-org/synapse,matrix-org/synapse,matrix-org/synapse,matrix-org/synapse