Thread confinement for Appender

[staticlibs]

Appender instances are not thread-safe. Only the close() method can be safely called from other threads concurrently with other operations.

append() and flush() operations operate on the same native buffer and cannot be called concurrently.

This PR implements a variant of a thread confinement for the Appender class intances. Only the thread that has created the Appender can call its methods (close() method still can be called from any thread). Method calls with throw SQLExeptions when called from other threads.

When it is necessary to use Appender from multiple threads, it is required to call unsafeBreakThreadConfinement() method first and use a Lock instance returned from it to synchronize the access to this Appender instance.

Example:

try (DuckDBAppender appender = connection.createAppender("tab1")) {
    Thread th = new Thread(() -> {
        // appender.flush(); // throws SQLException
        Lock appenderLock = appender.unsafeBreakThreadConfinement();
        appenderLock.lock();
        try {
            appender.flush();
        } catch (SQLException e) {
            e.printStackTrace();
        } finally {
            appenderLock.unlock();
        }
    });
    th.start();
    th.join();
}

Testing: a concurrent test added that, without the patch, was crashing the JVM in about 1 of 10 runs.

Fixes: #582