Add receive_nowait() to all receive streams

[agronholm]

Occasionally there is a use case where the code would like to receive all the data it can without waiting. The memory object stream has the receive_nowait() method for this purpose. I figured this could be easily added to socket streams too.

[agronholm]

This was inspired by redis's code doing this defensively to ensure that nothing was readable from the stream.

[agronholm]

We could add this method to the base classes as non-abstract but raising NotImplementedError, and have a subclass hook that emits a DeprecationWarning if a subclass doesn't implement this. Then in v5.0 we make it an abstract method. How does that sound?