Notifications in a distinct topic when starting/ending each file

[bleporini]

As it has been asked a couple of times, here is a proposal to implement notifications when files are processed. The basic idea is to add some SourceRecord during the poll when files are opened and closed.

I hope it helps, tell me if you see any case where it can make the task fail or if you see required improvement before merging.

[jcustenborder]

Hey @bleporini Well timed PR! I was going to start some work on this very soon. I'm going to merge this into another branch and base my changes on it! Thanks!