get(), pop(), popitem(), and setdefault() are not thread-safe

[crusaderky]

#92 left a few important methods of MutableMapping prone to race conditions.

get() may or may not be thread-safe depending on the implementation details in collections.abc.
pop(), popitem() and setdefault() are definitely not thread-safe at the moment of writing.

Worth noting that this does not impact dask/distributed; the exclusion of their usage is enforced by the distributed.spill.AsyncBufferProto API contract.