Serializable Snapshot Isolation (SSI)

[jeromegn]

This allows multiple concurrent write transactions.

Checklist

• Change MSRV back to 1.74
• Uncomment unwrap_used lint
• Remove #[inline] attributes
• Keep the option to use the single-writer transaction type
• Remove dependency on wmark (copy just the necessary parts into here), or maybe extract SnapshotTracker into a generic Watermark (probably needs a rewrite)
• Increment snapshot tracker watermark, so that old MVCC versions can be freed
• Verify that TxPartition does not create a cyclic Arc to TxKeyspace
• Clean up unused, commented code
• Look at TODO comments in write_tx
• More tests (ranges, prefix, fetch_update, update_fetch, etc.)
• Make sure memory usage has an upper bound (stuff is getting GC'ed in the transaction manager)
• Make CI pass
• Think about lifetime on Transaction (dangling iter)
• Benchmarks (WriteSet::insert was marked as SLOW function... cannot be sped up by SSI tx...)

[marvin-j97]

Thanks! I would give you the moon