Stability tweaks in the SWORD worker, fixes #4182

Changes introduced by this commit:
- The worker class destroys the ZSL object after a job is done to make
  sure that the PHP process doesn't keep the index locked.
- Keeps the MySQL connection alive by pinging the server every ~30 secs
  since AtoM's default db connection is persistent and they seem impossible
  to be closed.
- Stop forking the worker when a job starts, the memory leak seem to be
  solved now.

Squashed commit of the following:

commit e1ef03a95851b3ec32113553c01e706cb28de698
Author: Jesús García Crespo <jesus@sevein.com>
Date:   Tue Apr 2 00:43:56 2013 -0700

    Stop replicating the worker

    If I remember well, I used to fork the worker because the application
    was leaking memory. It's a lot better now and If we find more issues
    with memory we should be able to diagnose it with xdebug and fix it.

commit 4c395ab22a64118cc42b8aab59af6a3d4dbf2dd8
Author: Jesús García Crespo <jesus@sevein.com>
Date:   Tue Apr 2 00:35:15 2013 -0700

    Close index in the worker, refs #4182

commit 424350ae6397399f4314a0d8994557e142f99a5d
Author: Jesús García Crespo <jesus@sevein.com>
Date:   Tue Apr 2 00:28:04 2013 -0700

    Try to keep the db connection alive

    Instead of trying to close it. It seems impossible to close PDO
    connections when they are intended to be persistent. I'm not clear
    on what would happen if I try to restablish a connection that has
    timed out. Pinging the server periodically seems a reasonable
    solution for now.
Update gearman worker upstart service