`produces_range_tombstone()` is smart enough to not just try to read
one range tombstone from the input and compare it to the passed
reference, but to read as many range tombstones as the reader is
looking at (including none) using `may_produce_tombstones()` and
record those appropriately.
When `produces(const schema&, const mutation_fragment&)` is passed a
range tombstone as the second argument, it does not do anything
special -- it just reads one fragment, disregards it (!), and applies
its second argument to both "expected" and "encountered" range
tombstone lists. The right thing here is to use the same logic as
`produces_range_tombstone()`; upcoming memtable-related reader
changes (which result in more split range tombstones) cause some unit
tests to fail without fixing this.
Refactor the relevant logic into a private method (`apply_rt()`) and
use that in both places.
Signed-off-by: Michael Livshin <michael.livshin@scylladb.com>