Minor compression refactoring

[pdillinger]

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.

Test Plan: existing tests / CI