[performance] Avoid resolving addresses multiple times
Parts of commhistory (GroupManager) will send a stream of resolve requests with many duplicates. This used to be tolerable because requests were handled slowly in the event loop, but now that they have higher priority it has become a problem. I went for the simple solution of ignoring requests that are identical to still-active requests (same uids, same listener, same requireComplete flag). It's not optimal (we could combine more requests into the same backend query if we tried), but it solves the problem without adding a lot of bookkeeping.
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