Fix race condition and potential error in logfile deleting

[Digenis]

This is a bit of a [WIP] issue because there are more details to be added when I have the time

environ.py has the code responsible for deleting logfiles.

    def _get_file(self, message, dir, ext):
        logsdir = os.path.join(dir, message['_project'], \
            message['_spider'])
        if not os.path.exists(logsdir):
            os.makedirs(logsdir)
        to_delete = sorted((os.path.join(logsdir, x) for x in \
            os.listdir(logsdir)), key=os.path.getmtime)[:-self.jobs_to_keep]
        for x in to_delete:
            os.remove(x)
        return os.path.join(logsdir, "%s.%s" % (message['_job'], ext))

This deletes logs of hanged spiders.
When a spider hangs, it stops updating its log's mtime
and its log can be soon deleted by this code.
On windows, it should even crash because it doesn't allow open files to be deleted.

psutil can be used to check if the file is still open.
It is already being discussed as an optional dependency in scrapy (scrapy/scrapy#1329)

There is also a race condition between to_delete = sorted(...) and os.remove().
If something else removes the file, it will break. ENOENT should be ignored.