Investigate how to take in Facebook MyRocks blind delete

Upstream facebook has a feature implemented here : https://github.com/facebook/mysql-5.6/commit/f15fcf28391d6e222b26c2332e7c3c9ac90965fc

The feature allows MyRocks to skip looking up a row on deletion and simply drop a toumbstone for the row into RocksDB. 

This feature is somewhat intertwined with the writebatch feature that is also not yet imported (see MYR-47).

We need to ensure that the feature is complete and safe to use (will not cause cross index inconsistency).

This needs to be documented that Percona Server does not yet support this feature.