Summary:
Support a window of `active_width` keys that rolls through `[0, max_key)` over the duration of the test. Operations only affect keys inside the window. This gives us the ability to detect L0->L0 deletion bug (#2722).
Closes https://github.com/facebook/rocksdb/pull/2739

Differential Revision: D5628555

Pulled By: ajkr

fbshipit-source-id: 9cb2d8f4ab1a7c73f7797b8e19f7094970ea8749