Feature: Have the ability to configure a more comprehensive deltaContent

[mzgubin]

Currently when using deltaContent flag in the configurations the idea is that only content/nodes that have been created after the last run of grabbit will be copied over.
This presents a bit of an issue when something goes wrong and there is a failure.
When a failure occurs in the job run, the rest of the content that was supposed to be copied does not get copied.

And on subsequent job runs if the deltaContent flag is enabled what Grabbit does is only copy content after the last job run that contained that same path. So if the same job is retried it will skip over copying those nodes.

What would be great if we have the ability to specify another kind of deltaContent (maybe call it deepDeltaContent?) which would do a check on the target AEM instance to see if those files are there or not first instead of just comparing the timestamps.
I know this will tremendously slow down the copying, but might be really useful to get a comprehensive copy that ensures that all of the content does get copied over even if there were failures.