Use the same (web) worker for multiple data decode

[ivmartel]

The current code creates one (web) worker per worker thread run (see thread.js#L204). This code is mainly used when decoding jpeg encoded DICOM data and ends up creating one worker per data decode. This is inefficient since the worker setup can be costly (multiple imports and in turn http calls). The worker should be terminated only when the worker pool is not needed anymore.