sstableloader: Handle range tombstones better
sstableloader: Handle range tombstones better

References outbrain case of delete of rows matching subsets
of clustering columns. Cassandra generates slightly weird
tombstones for this.

Updated to try to detect the cases of "same-ended" min/max
ranges and extract the clutering deletion from this.

Also fixed so "where" is coalesqued more correctly, and
delete of cql rows can be merged. Less redundant statements.
Message-Id: <bd6805f27b30758a6569284d7853d3812dd1deee@scylladb.com>

scylladb/scylla-tools-java,scylladb/scylla-tools-java,scylladb/scylla-tools-java,scylladb/scylla-tools-java