Summary:
* Mostly, remove `sample_for_compression` from CompressionInfo because it's not used by the core function it serves, `CompressData()`. Confusing (and inefficient), especially in db_bench where it appears to use `FLAGS_sample_for_compression` in places where it is actually ignored.
* Various clarifying comments, clean-ups, and tiny optimizations
* Prepare some structures like `CompressionDict` for more usage
* Some TODOs and FIXMEs about some things I've noticed are amiss, confusing, or excessive
* A notable optimization opportunity that might become a "pay as you go" improvement for the potential indirection costs of customizable compression: use C++23's resize_and_overwrite() in compress functions to avoid zeroing the string buffer contents before populating it.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/13539
Test Plan: existing tests / CI
Reviewed By: hx235
Differential Revision: D73451273
Pulled By: pdillinger
fbshipit-source-id: 0373627466d695043d21146ce34d52f189ae9432
c368c6afe8