Change the collection overload of compaction_garbage_collector::collect()
to accept a collection_mutation (a serialized blob) instead of
collection_mutation_description (an owned, deserialized
representation).
Change collection_mutation_description::compact_and_expire() (the only
user) to build a serialized collection_mutation, with the new
collection_mutation_writer directly.
Allows dropping collection_mutation_description from the
compaction_garbage_collector interface, getting us one step closer to
removing it.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>