Test case test_reading_with_different_schemas uses randomly generated
pairs of mutations and tries to upgrade one to the schema of the other.
However, there are cases when one schema cannot be upgraded to another,
for example, counter and non-counter schemas.
This patch changes the type of the mutation partition's row_tombstones
to be a range_tombstone_list, so that they are now represented as a
set of disjoint ranges. All of its usages are updated accordingly.
Fixes#1155
Signed-off-by: Duarte Nunes <duarte@scylladb.com>
