Problem: crashed when trying to delete expired tickets

Solution: when ticket expires, there's no need to delete it, as
this is already done by zloop. Thus, clear our own ticket handle
when ticket expires.
Problem: client hands in destructor, on interrupt

As client destructor does a handshake with its background thread,
to allow a clean disconnect when possible, it hangs if the user
interrupted the process with Ctrl-C (as it never gets a reply
signal in s_accept_reply ()).

Solution: if zsys_interrupted is true, do not wait for a reply,
just return -1 immediately. The destructor then completes happily.
Merge pull request #183 from hintjens/master

Problem: crashed when trying to delete expired tickets