Added __slots__ to AsynCoroSocket and Coro for space savings.
SSL buffers read data. Until now I apparently didn't understand (probably
still don't) how this works/interfers with polling mechanism. It seemed to
work till now because I haven't tried with send_msg/recv_msg, which cause
SSL to buffer when reading only the lenght of message. Now we check if
there is pending data after registering with poller and then proceed to
read off pending data. With this, any new data arriving will be notified
which we read through poller.