TODO

Deal with version-mismatch-header received from the server

Cope with unknown frame types better. Currently it gets horribly
confused about frame lengths.

Make client brutal by default, killing the program on any amqp
error. Only if the user disables this behaviour will the user get to
deal with error conditions themselves. Make use of amqp_rpc_reply
consistent (i.e. universal), and rename it something like amqp_errno.