Queries with `ALLOW FILTERING` and constraints on counter
values used to be rejected as "unimplemented". The reason
was a missing tri-comparator, which is added in this patch.
Fixes#5635
* jul-stas-5635-filtering-on-counters:
cql/tests: Added test for filtering on counter columns
counters: add comparator and remove `unimplemented` from restrictions
(cherry picked from commit c32faee657)