Aggregate functions on counters do not exist. Until now counters
could, at best, fall back to blob->blob overloads, e.g.:
```
cqlsh> select max(cnt) from ks.tbl;
system.max(cnt)
----------------------
0x000000000000000a
(1 rows)
cqlsh> select sum(entities) from ks.tbl;
InvalidRequest: Error from server: code=2200 [Invalid query]
message="Invalid call to function sum, none of its type signatures match
[...]
```
Meanwhile, counters are compatible with bigints (aka. `long_type'),
so bigint overloads can be used on them (e.g. sum(bigint)->bigint).
This is achieved here by a special rule in overload resolution, which
makes `selector' perceive counters as an `EXACT_MATCH' to counter's
underlying type (`long_type', aka. bigint).
127e258ade