Summary:
Rather than storing a `vector<RangeTombstone>`, we now store a
`vector<RangeTombstoneStack>` and a `vector<SequenceNumber>`. A
`RangeTombstoneStack` contains the start and end keys of a range tombstone
fragment, and indices into the seqnum vector to indicate which sequence
numbers the fragment is located at. The diagram below illustrates an
example:

```
tombstones_:     [a, b) [c, e) [h, k)
                   | \   /  \   /  |
                   |  \ /    \ /   |
                   v   v      v    v
tombstone_seqs_: [ 5 3 10 7 2 8 6  ]
```

This format allows binary searching the tombstone list to use less key
comparisons, which helps in cases where there are many overlapping
tombstones. Also, this format makes it easier to add DBIter-like
semantics to `FragmentedRangeTombstoneIterator` in the future.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4632

Differential Revision: D13053103

Pulled By: abhimadan

fbshipit-source-id: e8220cc712fcf5be4d602913bb23ace8ea5f8ef0