partition_snapshot_row_cursor: Update change mark in prepare_heap
partition_snapshot_row_cursor: Update change mark in prepare_heap

The heap's iterators validity is checked with the change mark,
which is updated every time heap is recreated. Factor these
updates out and keep the mark together with the heap it protects.

Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@scylladb.com>

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