Tests for the RAW lock mechanism.

The volatile DB uses an abstraction we call a RAWLock, a lock that allows the following combinations of readers, appender and writer:

         │ Reader │ Appender │ Writer │
─────────┼────────┼──────────┼────────┤
Reader   │   V    │     V    │    X   │
Appender │░░░░░░░░│     X    │    X   │
Writer   │░░░░░░░░│░░░░░░░░░░│    X   │

It improves concurrent access. In the test we generate lots of threads, some readers, some appenders, some writers, each concurrently accessing some data protected by the lock. We then record the access pattern; the test would fail if at any point it would see a forbidden combination (for example, a writer and a reader both having access at the same time).