Applied this command to the code, making it a bit shorter and slightly
more readable.
```
cargo +nightly clippy --all-features --benches --tests --workspace --fix -- -A clippy::all -W clippy::uninlined_format_args
cargo +nightly fmt --all
```
* compress sstable with zstd
* add some details to sstable readme
* compress only block which benefit from it
* multiple changes to sstable
make compression optional
use OwnedBytes instead of impl Read in sstable, required for next point
use zstd bulk api, which is much faster on small records
* cleanup and use bulk api for compression
* use dedicated byte for compression
* switch block len and compression flag
* change default zstd level in sstable
* prepare fastfield format for null index
* add format version for fastfield
* Update fastfield_codecs/src/compact_space/mod.rs
* switch to variable size footer
* serialize delta of end
