clustering_ranges_walker: Make adjacency more accurate
clustering_ranges_walker: Make adjacency more accurate

Current check considered some adjacent range tombstones as overlapping
with the ranges. Making this more accurate will become more important
after we will rely on putting p_i_p::after_all_clustered_rows() in
_current_start in out-of-range state.

avikivity/scylla,avikivity/scylla,duarten/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla,duarten/scylla,duarten/scylla,scylladb/scylla,avikivity/scylla