service/storage_service.cc: Make effective_ownership preemptable
service/storage_service.cc: Make effective_ownership preemptable

A lot is going on when calculating effective ownership.
For each node in the cluster, we need to go over all the ranges belong
to that node and see if that node is the owner or not.

This patch uses futurized loops with do_for_each so it would preempt if
needed.

The patch replaces the current for-loops with do_for_each and do_with
but keeps the logic.

Fixes #6380

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