This fixes a long standing bug related to handling of non-full
clustering keys, issue #1446.
after_key() was creating a position which is after all keys prefixed
by a non-full key, rather than a position which is right after that
key.
This will issue will be caught by cql_query_test::test_compact_storage
in debug mode when mutation_partition_v2 merging starts inserting
sentinels at position after_key() on preemption.
It probably already causes problems for such keys.
23e4c83155