many syncv3 optimisations; database, RocksDB, and misc performance improvements; refactoring and cleanup; fix private read receipt support; and some more bug fixes

[x86pup]

• syncv3 was further heavily optimised, greatly improving client user experience
• more general performance improvements, parallelism, and more asyncification was done
• further RocksDB tuning was done to heavily improve database performance, and bump RocksDB to v9.8.4
• misc logging and tracing improvements
• we pass some more complement tests now
• bump nix flake lockfile after like 3 months, and remove libolm dependency (previously required by complement, not in any way used by conduwuit)
• improve performance of updating your profile picture, displayname, etc
• fix long-standing problem where membership updates on updating your profile would still copy/reuse some fields that shouldn't be reused/copied, such as the reason or is_direct flag
• improve and refactor federation membership endpoint checks
• update welcome text on new conduwuit deployments
• copy/implement Synapse compatibility behaviour where POST requests that have no JSON body are tolerated and assumed to be {}. some older clients/libraries/bridges/etc such as matterbridge rely on this (Fix compatibility with conduit/conduwuit 42wim/matterbridge#2199)
• add some missing required checks with account data
• fix MSC4133/MSC4175 timezone key being sent twice on full profile requests
• fix inherited non-functional private read receipt support. private read receipts were never returned over /sync, and clients that rely on receiving them over /sync such as gomuks web now have functional counters and properly mark rooms as read
• enable production RocksDB track WAL in MANIFEST config option for improved database reliability and recovery at no cost/expense (https://github.com/facebook/rocksdb/wiki/Track-WAL-in-MANIFEST)
• add config option for RocksDB paranoid file checks for further improved database reliability at a slight performance cost (https://github.com/facebook/rocksdb/wiki/Online-Verification#columnfamilyoptionsparanoid_file_checks)

also im tired rn 🥴 i hope i got it all