ShellCmd backend never checks for stale resource files

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What steps will reproduce the problem?
1. Start running jobs using the `shellcmd` backend
2. Interrupt GC3Pie while the jobs are still in RUNNING state
3. Repeat from step 1. until all available slots are consumed (`max_cores`)

What is the expected output? What do you see instead?

Execution slots are allocated but never claimed back.  It seems that
the `shellcmd` backend only checks that a PID file is stale when the
job status is updated; if the job is not part of the current session,
it is never updated...

We should probably run a check for stale PIDs when the backend is
initialized.  I am not sure however, what happens if a resource file
pertaining to another session is deleted.

Original issue reported on code.google.com by [e-mail] on 12 Jun 2015 at 10:40