fixes related to room v12 and sytest

[FrenchGithubUser]

Discovered while writing matrix-org/sytest#1429

Those might be split into their separate PRs if necessary.

• Fix third-party (3pid) invites over federation failing intermittently in version 12 rooms, whose room IDs no longer encode a server name.
• Fix /createRoom intermittently failing with a 500 error in version 12 rooms when the same user creates several rooms at once, due to colliding room IDs.

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