In this patch we add
token_metadata::set_topology_transition_state method.
If the current state is
write_both_read_new update_pending_ranges
will compute new ranges for read requests. The default value
of topology_transition_state is null, meaning no read
ranges are computed. We will add the appropriate
set_topology_transition_state calls later.
Also, we add endpoints_for_reading method to get
read endpoints based on the computed ranges.
0e4e2df657