Rewritten the I/O loop. All socket operations are now safe, meaning that

Rewritten the I/O loop. All socket operations are now safe, meaning that
you can delete the socket from anywhere in the hooks and nothing should break.
Also, the receive/transmit buffers are now regular xmalloc()'ed buffers,
not separate resources which would need shuffling around between pools.

sk_close() is gone, use rfree() instead.