Mechanism to discard a document

[aakarshg]

If a document has a key discard set to true, irrespective of what the doc contains it should be discarded. This can be helpful in cases where although the object creation is successful and schema validation suceeds, the doc might not have enough data thats useful.

[aakarshg]

https://review.gerrithub.io/#/c/redhat-performance/scribe/+/427932/ brings this functionality