Fixed issue #323

- also cleaned up zombie timer code on zloop; there was no need to
  allocate a block to store an integer, this could sit on the list
  item directly, with a suitable cast.

- minor cleanups to zframe.
Fixed deadlock in zbeacon and zmonitor

- if interrupted, background task did not handle TERMINATE
- general model for background tasks is to ignore zctx_interrupted and
  to work only off API commands.
New zproxy was mishandling multipart messages
Merge pull request #375 from hintjens/master

Fixed issue #323