This is the initial implementation of [this spec](https://docs.google.com/document/d/1X6pARlxOy6KRQ32JN8yiGsnWA9Dwqnhtk7kMDo8m9pI/edit).
* the topology version (int64) was introduced, it's stored in topology table and updated through RAFT at the relevant stages of the topology change algorithm;
* when the version is incremented, a `barrier_and_drain` command is sent to all the nodes in the cluster, if some node is unavailable we fail and retry indefinitely;
* the `barrier_and_drain` handler first issues a `raft_read_barrier()` to obtain the latest topology, and then waits until all requests using previous versions are finished; if this round of RPCs is finished the topology change coordinator can be sure that there are no requests inflight using previous versions and such requests can't appear in the future.
* after `barrier_and_drain` the topology change coordinator issues the `fence` command, it stores the current version in local table as `fence_version` and blocks requests with older versions by throwing `stale_topology_exception`; if a request with older version was started before the fence, its reply will also be fenced.
* the fencing part of the PR is for the future, when we relax the requirement that all nodes are available during topology change; it should protect the cluster from requests with stale topology from the nodes which was unavailable during topology change and which was not reached by the `barrier_and_drain()` command;
* currently, fencing is implemented for `mutation` and `read` RPCs, other RPCs will be handled in the follow-ups; since currently all nodes are supposed to be alive the missing parts of the fencing doesn't break correctness;
* along with fencing, the spec above also describes error handling, isolation and `--ignore_dead_nodes` parameter handling, these will be also added later; [this ticket](https://github.com/scylladb/scylladb/issues/14070) contains all that remains to be done;
* we don't worry about compatibility when we change topology table schema or `raft_topology_cmd_handler` RPC method signature since the raft topology code is currently hidden by `--experimental raft` flag and is not accessible to the users. Compatibility is maintained for other affected RPCs (mutation, read) - the new `fencing_token` parameter is `rpc::optional`, we skip the fencing check if it's not present.
Closes#13884
* github.com:scylladb/scylladb:
storage_service: warn if can't find ip for server
storage_proxy.cc: add and use global_token_metadata_barrier
storage_service: exec_global_command: bool result -> exceptions
raft_topology: add cmd_index to raft commands
storage_proxy.cc: add fencing to read RPCs
storage_proxy.cc: extract handle_read
storage_proxy.cc: refactor encode_replica_exception_for_rpc
storage_proxy: fix indentation
storage_proxy: add fencing for mutation
storage_servie: fix indentation
storage_proxy: add fencing_token and related infrastructure
raft topology: add fence_version
raft_topology: add barrier_and_drain cmd
token_metadata: add topology version
e41ff4604d