Before this commit, "compression options" written into
CompressionInfo.db (and used to construct a decompressor)
have a 1:1 correspondence to "compression options" specified
in the schema.
But we want to add a new "compression option" -- the compression
dictionary -- which will be written into CompressionInfo.db
and used to construct decompressors, but won't be specified in the
schema.
To reconcile that, in this commit we introduce the notion of a "hidden
option". If an option name in `CompressionInfo.db` begins with a dot,
then this option will be used to construct decompressors, but won't
be visible for other uses. (I.e. for the `sstable_info` API call
and for recovering a fake `schema` from `CompressionInfo.db` in the
`scylla sstable` tool).
Then, we introduce the hidden `.dictionary.{0,1,2,..}` options,
which hold the contents of the dictionary blob for this SSTable.
(The dictionary is split into several parts because the SSTable
format limits the length of a single option value to 16 bits,
and dictionaries usually have a length greater than that).
This commit only introduces helpers which translate dictionary blobs
into "options" for CompressionInfo.db, and vice-versa, but it doesn't
use those helpers yet. They will be used in later commits.
dd932ebb2f