Initial support for dxfs poll-based refresh
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.
Pylint fixes
Defer destruction of the old filesystem tree until after expensive API call

This minimizes the time in which the filesystem is visible in an inconsistent state during refresh.
Fix logic error left in dx close after adding multi-arg support