storage_proxy: switch to the exponential sharder for nonsingular queries
storage_proxy: switch to the exponential sharder for nonsingular queries

Nonsingular queries used exponential expansion of the token space to
avoid spending too much cpu time on near-empty tables, but the generation
of the search space was itself exponential.  Switch to the exponential sharder
which has linear cost.

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