LibsqlConnection state is not synchronized #105

#89 serializes statement close and execute with a statement lock. The connection fields baton, baseUrl, autoCommit, closed, and the cached metadata are plain fields with no synchronization. sqlite-jdbc serializes every call on its DB object.

Failing interleavings:

• Two statements on one non-auto-commit connection read the same baton. One request fails as a reused baton, and the last response overwrites the stream state.
• commit() runs between another thread's ensureTransactionStarted and its send.
• close() races executeStatement, so a stream can stay open.

The statement lock does not order calls into the connection. The statement never calls the connection while it holds its lock, except through checkClosed, which takes no lock. A connection-level lock therefore adds no lock-ordering risk.

Expected:

• Hold one connection-level lock around each pipeline round trip and around setAutoCommit, commit, rollback, and close.
• Add a concurrency test with two threads on one non-auto-commit connection.