Add a timeout to the CommandLineExecutorService

When a command line is run and freezes, the Nuxeo thread that started it also is freezed.

I had an example using wkhtmltopdf, when converting a complex distant page (nuxeo.com ) to pdf. Sometime, it could freeze on Linux Ubuntu (top shows wkhtmltopdf present, almost not using cpu, but never quitting.

So, by default there should be no timeout (compatibility), but a parameter in the XML configuration should allow to set a timeout (milliseconds or seconds probably). When this timeout is reached, if the command is still running, it is quit or possibly force quit.

Having a frozen command line is problem for example in async converters stacked in a limited queue. Once 2, 3, n ("max in queue") are frozen, no one can be added to the queue.