Add channel to worker for done work

[sa7mon]

Instead of the worker having to handle writing to the database or outputting logs to the terminal, the worker should only:

• take in buckets from input channel
• scan them
• push them into output channel

then s3scanner.Run() should wait for buckets to be pushed into the output channel and handle writing to the database and writing logs.

I think this is what "clean code" is referring to.

We'll need to add tests to verify the output channel is closed at appropriate times and the program exits gracefully. Once we make this change, we should be able to combine the 2 worker classes into a unified worker.