Add explicit cast to zhash_cursor calls

Problem: The zhash_cursor function returns const void * and the
zproto_codec_c assumes that it return const char *.

Solution: Add explicit cast to zhash_cursor calls.
Merge pull request #173 from danriegsecker/master

Add explicit cast to zhash_cursor calls
Revert "Add explicit cast to zhash_cursor calls"
Merge pull request #174 from zeromq/revert-173-master

Revert "Add explicit cast to zhash_cursor calls"
Merge pull request #175 from hintjens/master

Problem: need send/recv msg capability in client API
Problem: Client does not destroy chunk, frame, and msg properties
Solution: Destroy them in $(class.name)_destroy
Merge pull request #176 from jemc/master

Problem: Client does not destroy chunk, frame, and msg properties
Problem: caller should own message returned by _recv

Solution: do not destroy in generated code; allow caller to do this.

This is more consistent with what we'd expect from a CLASS API and
does not cost anything in performance terms. Also since the caller
will usually consume the message in order to process it, it also
makes sense.