Equivalent to abstract_replication_strategy get_range_addresses,
yet synchronous, as it uses the precalculated map.
Call it from storage_service::get_new_source_ranges
and range_streamer::get_all_ranges_with_sources_for.
Consequently, get_new_source_ranges and removenode_add_ranges
can become synchronous too.
Unfortunately we can't entirely get rid of
abstract_replication_strategy::get_range_addresses
as it's still needed by
range_streamer::get_all_ranges_with_strict_sources_for.
Signed-off-by: Benny Halevy <bhalevy@scylladb.com>