alternator: add filtering for key equality
alternator: add filtering for key equality

Until now, filtering in alternator was possible only for non-key
column equality relations. This commit adds support for equality
relations for key columns.

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