Serializable Snapshot Isolation (SSI)

[jeromegn]

This allows multiple concurrent write transactions.

Checklist

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