Do not reuse global worker pool for files

[trowski]

This modifies the parallel driver to create its own internal pool by default, rather than reuse the global worker pool. A specific pool can still be passed to the constructor (though I assume this use is rather rare).

The AMP_WORKER constant is no longer used to prevent the blocking driver from being used. This way, a Task may still use async file access and get the behavior expected.

/cc @azjezz