All zauth commands are now synchronous

This means caller waits for signal (zsock_wait()) after each command,
which ensures actor finishes its work before the calling thread continues.
Otherwise, the caller will run ahead and attempt to use actor functionality
that isn't ready yet. zproxy works the same way.
Rewrote zproxy class to use zactor model

Existing API is deprecated and moved to zproxy_v2. The new actor based
API is slightly more verbose than the old one. Sorry about this.
Merge pull request #597 from hintjens/master

zproxy re-implemented as actor