partition_snapshot_reader: do not accidentally copy schema
partition_snapshot_reader: do not accidentally copy schema

Functions `upper_bound` and `lower_bound` had signatures:
```
template<typename T, typename... Args>
static rows_iter_type lower_bound(const T& t, Args... args);
```
This caused a dacay from `const schema&` to `schema` as one of the args,
which in turn copied the schema in a fair number of the queries. Fix
that by setting the parameter type to `Args&&`, which doesn't discard
the reference.

Fixes #9502

Closes #9507

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