GIL release for perspective-python Async mode

[texodus]

Adds set_loop_callback() public method to the PerspectiveManager class, which takes as an argument a loop-queueing function, such as asyncio call_soon() and Tornado add_callback(). Once invoked, Perspective will presume to be owned by the calling thread, and will release the Python GIL for the bulk of reading/writing Apache Arrow data, as well as calculating
new View contexts. There are some new restrictions in this mode, namely that Perspective is no longer thread safe, covered in the documentation.

[sc1f]

Looks good! Tested locally & offline and enables some really great parallel performance over large datasets.