This series adds a global read barrier to raft_group0_client, ensuring that Raft group0 mutations are applied on all live nodes before returning to the caller.
Currently, after a group0_batch::commit, the mutations are only guaranteed to be applied on the leader. Other nodes may still be catching up, leading to stale reads. This patch introduces a broadcast read barrier mechanism. Calling send_group0_read_barrier_to_live_members after committing will cause the coordinator to send a read barrier RPC to all live nodes (discovered via gossiper) and waits for them to complete. This is best effort attempt to get cluster-wide visibility of the committed state before the response is returned to the user.
Auth and service levels write paths are switched to use this new mechanism.
Fixes https://scylladb.atlassian.net/browse/SCYLLADB-650
Backport: no, new feature
Closesscylladb/scylladb#28731
* https://github.com/scylladb/scylladb:
test: add tests for global group0_batch barrier feature
qos: switch service levels write paths to use global group0_batch barrier
auth: switch write paths to use global group0_batch barrier
raft: add function to broadcast read barrier request
raft: add gossiper dependency to raft_group0_client
raft: add read barrier RPC
37aeba9c8c