Use directory server for room joins (#3899)

When we do a join, always try the server we used for the alias lookup first.

Fixes #2418

changelog.d/3899.bugfix

@@ -0,0 +1 @@
+When we join a room, always try the server we used for the alias lookup first, to avoid unresponsive and out-of-date servers.

synapse/handlers/room_member.py

@@ -583,6 +583,11 @@ class RoomMemberHandler(object):
         room_id = mapping["room_id"]
         servers = mapping["servers"]
 
+        # put the server which owns the alias at the front of the server list.
+        if room_alias.domain in servers:
+            servers.remove(room_alias.domain)
+        servers.insert(0, room_alias.domain)
+
         defer.returnValue((RoomID.from_string(room_id), servers))
 
     @defer.inlineCallbacks