Major restructuring of how streams work. Neither input nor output
streams are now blocking. This avoids stalling the rest of the client or
server when a peer is slow or unresponsive.

Note that this puts an extra burden on users of streams to make sure
they are allowed to do their work once the underlying transports are
ready (e.g. monitoring fds).

External callers don't need to know the exact details, only if there is
data that needs to be flushed or not.

This allows us to handle peaks in input and output streams gracefully
without having to block processing.

Just have a simply number of bytes argument to avoid a lot of
complexity.

Most streams are backed by a memory buffer. Create common base classes
for this functionality to avoid code duplication.