Start transaction before prepare call

[staticlibs]

Previously, when auto-commit if off, the implicit transaction was started automatically when PreparedStatement#execute(void) is called. This was found to cause the problems with
Statement#executeQuery(String), that calls prepare(String) and execute() internally. As transaction was starting only after prepare call - the prepare itself was running with effective auto-commit mode (in a separate implicit transaction). It is harmless in most cases, but was found to break transaction-wide caching in duckdb-iceberg.

The problem was only happening when the execute(String) is running outside of active transaction, for example as a first call in freshly opened connection. Still this was causing non-negligible overhead.

This change makes the implicit transaction to be started before prepare call, so the same transaction is active during execute call.

There are no changes in logic when auto-commit is on (default per JDBC spec), so the Iceberg caching is still broken in this mode. This is intended to be fixed in future, for now to have effective caching with duckdb-iceberg it is necessary to disable auto-commit, it can be done by one of the following methods:

In connection config:

Properties config = new Properties();
config.put(DuckDBDriver.JDBC_AUTO_COMMIT, false);
Connection conn = DriverManager.getConnection(url, config);

In connection URL:

Connection conn = DriverManager.getConnection("jdbc:duckdb:path/to/my.db;jdbc_auto_commit=false;");

On a connection manually:

conn.setAutoCommit(false);

Testing: there are no easily observable effects from this change, the transactional logic is covered by existing tests, Iceberg caching is tested manually.