sstables: Add support for writing range tombstones in SSTables 3.x format.
sstables: Add support for writing range tombstones in SSTables 3.x format.

For SSTables 3.x. ('mc' format), range tombstones are represented by
their bounds that are written to the data file as so-called RT markers.
For adjacent range tombstones, an RT marker can be of a 'boundary' type
which means it closes the previous range tombstone and opens the new
one.

Internally, sstable_writer_m relies on range_tombstone_stream to both
de-overlap incoming range tombstones and order them so that when they
are drained they can be easily thought of as just pairs of their bounds.

duarten/scylla,avikivity/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla,duarten/scylla,duarten/scylla