Summary:
Logically, subcompactions process a key range [start, end); however, the way
this is currently implemented is that the `CompactionIterator` for any given
subcompaction keeps processing key-values until it actually outputs a key that
is out of range, which is then discarded. Instead of doing this, the patch
introduces a new type of internal iterator called `ClippingIterator` which wraps
another internal iterator and "clips" its range of key-values so that any KVs
returned are strictly in the [start, end) interval. This does eliminate a (minor)
inefficiency by stopping processing in subcompactions exactly at the limit;
however, the main motivation is related to BlobDB: namely, we need this to be
able to measure the amount of garbage generated by a subcompaction
precisely and prevent off-by-one errors.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8327

Test Plan: `make check`

Reviewed By: siying

Differential Revision: D28761541

Pulled By: ltamasi

fbshipit-source-id: ee0e7229f04edabbc7bed5adb51771fbdc287f69