async: resolve send/recv events immediately, if possible
async: resolve send/recv events immediately, if possible

If async messages can be sent/recvd at call time,
process them immediately rather than putting them on the queue to be polled.

Roughly 5x performance improvement in benchmarks.

This does open the door for busy sockets to starve other events.

swn1/pyzmq,swn1/pyzmq,swn1/pyzmq