dht: fix partitioner initialization for tests
dht: fix partitioner initialization for tests

The partitioners now depend on smp::count to be initialized correctly,
but smp::count isn't available at static initialization time.

The scylla executable isn't affected because it calls set_global_partitioner()
after smp::count has been initialized.

Fix by deferring initialization to the first global_partitioner() call.

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