Move time-consuming activities off of the event loop thread

Currently operations like serialization/deserialization, disk I/O, and compression all take place on the event loop thread. This is convenient because it avoids complex issues that can arise from concurrency. However when users pass around large pieces of data this can block the event loop for a non-trivial period which can damage the smoothness of inter-node communication.

So in some cases we might want to move these processes off-thread using a `concurrent.futures.ThreadPool`. I did this a while ago here: https://github.com/dask/distributed/pull/947 . It might be time to reconsider and expand that work.

cc @pitrou