locator: Add yield in do_get_ranges and friends
locator: Add yield in do_get_ranges and friends

Not all calculate_natural_endpoints implementations respect can_yield
flag, for example, everywhere_replication_strategy.

This patch adds yield at the caller site to fix stalls we saw in
do_get_ranges.

Fixes #8943

Closes #9139

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