Add execution timeout to Command Extractor

[qkaiser]

Fix #621

[e3krisztian]

I think it solves a concrete problem, so I do not request bigger immediate changes, but I am slightly unhappy about it:

• 12 hours is too much for a single extractor, I would expect them to be fast, and would prefer a 10 minute limit instead, if that limit is not enough, we should definitely look at what is wrong - do we know of something slow, and non-problematic?
• the limit is not configurable
• it is in a need of a test (monkey-patching a sub-second timeout and sleep 60 as external command would work)
• it is only for external commands?

[qkaiser]

I think it solves a concrete problem, so I do not request bigger immediate changes, but I am slightly unhappy about it:

• 12 hours is too much for a single extractor, I would expect them to be fast, and would prefer a 10 minute limit instead, if that limit is not enough, we should definitely look at what is wrong - do we know of something slow, and non-problematic?

Extraction time is dependent on the input file size. We chose this value because the objective is to make sure unblob stops at some point, not to spot slow extractors.

• the limit is not configurable

Yes, I don't think we want it to be at this point.

• it is in a need of a test (monkey-patching a sub-second timeout and sleep 60 as external command would work)

I can add it to this branch, what do you say ?

• it is only for external commands?

Yes, endless loops have been observed in external commands. When that happens in our own code we can fix it :)