HS apparently doesn't block when clients provision an AS-backed virtual room (SYN-497)

[matrixbot]

Submitted by @​matthew:matrix.org jfred reports whilst building matrix-xmpp-bridge that when a client asks to join a non-existent alias backed by an AS, the HS doesn't block until the AS has created the room. As a result, the client sees a 404

(Imported from https://matrix.org/jira/browse/SYN-497)