The functions are called from RESful API so has to return ips for backwards
compatibility, but internally we can use host ids as long as possible
and convert to ips just before returning. This also drops usage of ip
based erm function which we want to get rid of.
locator/util.hh already has get_range_to_address_map which is exactly
like the one in the storage_service. So remove the later one and use the
former instead.
`describe_ring()` was implemented as a method of `storage_service`. This
patch extracts it from there to a standalone helper function in
`locator/util.hh`.
