Disable FUSE threading in dxfuse driver.

Multiple threads accessing the same DXFile threadpool confuse our (not particularly smart) buffer and readahead logic. Moreover, operation of the same threadpool from multiple consumer threads is not safe and leads to unexpected results as well as crashes (e.g. "ValueError: generator already executing"). We should investigate making this safe (via locks or per-consumer threadpools) or detecting it and crashing early.