mutation_partition: Trim range tombstones to query ranges
mutation_partition: Trim range tombstones to query ranges

Current code was only selecting overlapping range tombstones.

We will need range tombstones to be trimmed. This is needed to change
the semantics of flat_mutation_reader v1 to produce only range
tombstones trimmed to clustering restrictions. This constructor is
used in unit tests which verify what reader produces.

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla