When new nodes are added or existing nodes are deleted, the topology
state machine needs to shunt reads from the old nodes to the new ones.
This happens in the `write_both_read_new` state. The problem is that
previously this state was not handled in any way in `token_metadata` and
the read nodes were only changed when the topology state machine reached
the final 'owned' state.
To handle `write_both_read_new` an additional `interval_map` inside
`token_metadata` is maintained similar to `pending_endpoints`. It maps
the ranges affected by the ongoing topology change operation to replicas
which should be used for reading. When topology state sm reaches the
point when it needs to switch reads to a new topology, it passes
`request_read_new=true` in a call to `update_pending_ranges`. This
forces `update_pending_ranges` to compute the ranges based on new
topology and store them to the `interval_map`. On the data plane, when a
read on coordinator needs to decide which endpoints to use, it first
consults this `interval_map` in `token_metadata`, and only if it doesn't
contain a range for current token it uses normal endpoints from
`effective_replication_map`.
Closes#13376
* github.com:scylladb/scylladb:
storage_proxy, storage_service: use new read endpoints
storage_proxy: rename get_live_sorted_endpoints->get_endpoints_for_reading
token_metadata: add unit test for endpoints_for_reading
token_metadata: add endpoints for reading
sequenced_set: add extract_set method
token_metadata_impl: extract maybe_migration_endpoints helper function
token_metadata_impl: introduce migration_info
token_metadata_impl: refactor update_pending_ranges
token_metadata: add unit tests
token_metadata: fix indentation
token_metadata_impl: return unique_ptr from clone functions
7d9ab44e81