Fast column flush for rescan

[GalRogozinski]

Description

Currently when flushing a column family we iterate over its contents one-by-one
This is slow and maybe by using dropColumnFamily we can make this faster

Motivation

Accelerate the deletion phase in rescan/revalidate

Requirements

1. Fast delete column family

Notes

I initially wanted to implement this for #1482, but apparently it required more changes than I planned.
I believe we should implement this when we have a good idea about the persistence layer new design (#810)