Confirm that a socket has not been closed and does not have unread data at checkout

Because Ruby can allow threads to be killed explicitly by an application, it's possible that a socket can be written to and then never read from before being returned to the pool. It is therefore possible for a socket to be checked out and then read a killed thread's data.

Python uses poll or select to check that a socket has not been closed and also checks for unread data. We should do the same.

Here is the relevant Python code:

https://github.com/mongodb/mongo-python-driver/blob/master/pymongo/network.py#L169-L182

https://github.com/mongodb/mongo-python-driver/blob/master/pymongo/pool.py#L654-L677