Pekka Enberg 45288b1297 core/mvcc: Eliminate RwLock wrapping Transaction ...

The write and read sets in Transaction use SkipSet, which is thread-safe.
Therefore, drop the RwLock wrapping Transaction everywhere, increasing
MVCC throughput by almost 30%.

Before:

```
Running write throughput benchmark with 1 threads, 1000 batch size, 1000 iterations, mode: Mvcc
Database created at: write_throughput_test.db
Thread 0: 1000000 inserts in 6.50s (153927.21 inserts/sec)

=== BENCHMARK RESULTS ===
Total inserts: 1000000
Total time: 6.50s
Overall throughput: 153758.85 inserts/sec
Threads: 1
Batch size: 1000
Iterations per thread: 1000
```

After:

```
Running write throughput benchmark with 1 threads, 1000 batch size, 1000 iterations, mode: Mvcc
Database created at: write_throughput_test.db
Thread 0: 1000000 inserts in 5.10s (195927.13 inserts/sec)

=== BENCHMARK RESULTS ===
Total inserts: 1000000
Total time: 5.11s
Overall throughput: 195663.94 inserts/sec
Threads: 1
Batch size: 1000
Iterations per thread: 1000
```

2025-09-11 20:31:19 +03:00

..

mod.rs

core/mvcc: Eliminate RwLock wrapping Transaction

2025-09-11 20:31:19 +03:00

tests.rs

core/mvcc: Eliminate RwLock wrapping Transaction

2025-09-11 20:31:19 +03:00