This is an attempt to better cache the cases where there are a large
number of extremities to resolve over, which keep slightly changing.
This spawns from seeing issues on matrix.org.
We already have a cache over the exact state groups being resolved.
However, we can do better by caching the inputs into state res (i.e. the
conflicted sets), which are more likely to be constant across repeated
state res in a room. We key this cache based on a sha256 hash, on the
assumption that this will never conflict.
Also includes a commit that removes needless copying of the state.
This was correctly handled for the "fallback" case where the background
updates hadn't finished
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Co-authored-by: Eric Eastwood <erice@element.io>
Erik Johnston