Support for variable framesize in VST code

[jvxgit]

Audacity supports VST plugins. But it does so by passing in buffers with variable framesize. There are two approaches which may be followed:

1. Collect data and run processing only if there is enough data in memory. We do not change threads. This is not goog for real-time processing as it may result in peak loads.
2. Collect data and run processing in a second thread. This, however, would not work well in batch processing.
   For this issue, extend the CjvxFlexibleConnection class to run buffered and multithreaded.