Problem: zsock_send/recv are too slow

These methods, while cute, do a lot of heap work. This isn't as
fast as it could be. It's fine for controlling an actor via its
pipe. It's not good for very high volume work.

Solution: add "nasty" binary send/recv that work better for high
volume traffic. zsock_bsend/zsock_brecv. Note these store their
context in the socket object, so received strings are overwritten
on each zsock_brecv call.
Problem: zgossip does not work with zproto #147

The pull request https://github.com/zeromq/zproto/pull/147 breaks
existing zproto apps.

Solution: fix zgossip to work with this pull request and the new
v2 codec.
Problem: in some cases we need the actor socket handle

Specifically, zsock needs this when it's doing a bsend/brecv so
that it knows where to stick its cache.

Solution: add a zactor_sock() method.
Merge pull request #798 from hintjens/master

Added binary send/recv to zsock