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cfe69e057f
Note: this commit is meant to be a code refactoring only and is not intended to change the observable behaviour. Today `schema` contains a `compression_parameters`. `compression_parameters` contains an instance of `compressor`, and SSTable writers just share that instance. This is fine because `compressor` is a stateless object, functionally dependent on the schema. But in later parts of the series, we will break this functional dependency by adding dictionaries to compressors. Two writers for the same schema might have different dictionaries, so they won't be able to just share a single instance contained in the schema. And when that happens, having a `compressor` instance in the `schema`/`compression_parameters` will become awkward, since it won't be actually used. It will be only a container for options. In addition, for performance reasons, we will want to share some pieces of compressors across shards, which will require -- in the general case -- a construction of a compressor to be asynchronous, and therefore not possible inside the constructor of `compression_parameters`. This commit modifies `compression_parameters` so that it doesn't hold or construct instances of `compressor`. Before this patch, the `compressor` instance constructed in `compression_parameters` has an additional role of validating and holding compressor-specific options. (Today the only such option is the zstd compression level). This means that the pieces of logic responsible for compressor-specific options have to be rewritten. That ends up being the bulk of this commit.