sstableloader: Support range boundary tombstones
sstableloader: Support range boundary tombstones

Add support for range tombstone markers of RangeTombstoneBoundaryMarker
type, which consist of both start and end range bound:

{
  "type" : "range_tombstone_boundary",
  "start" : {
    "type" : "exclusive",
    "clustering" : [ "4" ],
    "deletion_info" : { ... }
  },
  "end" : {
    "type" : "inclusive",
    "clustering" : [ "4" ],
    "deletion_info" : { ... }
  }
}

Previously only RangeTombstoneBoundMarker was properly handled and if
SSTable contained a boundary range tombstone sstableloader crashed, as
it only processed end of boundary and omitted start.

Fixes #205.

Reviewed-by: Calle Wilund <calle@scylladb.com>
Reviewed-by: Piotr Jastrzebski <piotr@scylladb.com>

Closes #206

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