Add a mutation_fragment_stream_validating_filter to
sstables::writer_impl and use it in sstable_writer to validate the
fragment stream passed down to the writer implementation. This ensures
that all fragment streams written to disk are validated, and we don't
have to worry about validating each source separately.
The current validator from sstable::write_components() is removed. This
covers only part of the write paths. Ad-hoc validations in the reader
implementations are removed as well as they are now redundant.
cb4d92aae4