[FIX] workers: process signals during graceful shutdown to allow force shutdown

Required after e17844c946 which
enabled graceful shutdown of workers, including the cron worker.
The latter may typically be busy on long-running tasks that
will not be aborted by a simple graceful shutdown request.

A typical shutdown sequence initiated by a daemon manager such
as start-stop-daemon will involve multiple SIGTERM signals to
ensure the process really stops in a timely manner.
This was not possible after e17844c946
if any cron worker was busy.

openerp/service/workers.py

@@ -186,6 +186,11 @@ class Multicorn(object):
             for pid in self.workers.keys():
                 self.worker_kill(pid, signal.SIGINT)
             while self.workers and time.time() < limit:
+                try:
+                    self.process_signals()
+                except KeyboardInterrupt:
+                    _logger.info("Forced shutdown.")
+                    break
                 self.process_zombie()
                 time.sleep(0.1)
         else: