Add stop callback on shutdown

[qinghuangchan]

In this PR, I propose to add some additional handling when exiting.

Background for needing this feature:

I've opted to store my data in pgsql. The current pgsql insertion implementation doesn't do any caching, so i've monkey patched my own caching methods in.

However, when restarting the collector, the data in cache is lost, since there is no opportunity to do clean up.

I've added a stop method to the Callback class, as well as created a new BackendCallback base class which all other backend callbacks inherit.

The purpose of adding such handling is to allow the many types of callbacks to do some clean up where necessary.

In the run method of FeedHandler, I've added a finally clause to the try-except statement. This is where the code will try to call stop for all its feeds.

Im not too sure if you'd like my implementation - please let me know if there's something I can change to make it better.

• - Tested
• - Changelog updated
• - Contributors file updated (optional)

[bmoscon]

rather than put in an un-used stub, why not just check if stop is a defined method on callback before calling it? This would remove the need for all the added inheritance

[bmoscon]

move this under the current dev version, 1.6.2

[qinghuangchan]

done.

[qinghuangchan]

That's looks nicer indeed. changes committed.