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* Add a per-room mutex to federationapi when processing transactions
This has numerous benefits:
- Prevents us doing lots of state resolutions in busy rooms. Previously, room forks would always result
in a state resolution being performed immediately, without checking if we were already doing this in
a different transaction. Now they will queue up, resulting in fewer calls to `/state_ids`, `/g_m_e`, etc.
- Prevents memory usage from growing too large as a result and potentially OOMing.
And costs:
- High traffic rooms will be slightly slower due to head-of-line blocking from other servers,
though this has always been an issue as roomserver has a per-room mutex already.
* Fix unit tests
* Correct mutex lock ordering
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