Add a new virtual table `system.raft_state` that shows the currently
operating Raft configuration for each present group. The schema is the
same as `system.raft_snapshot_config` (the latter shows the config from
the last snapshot). In the future we plan to add more columns to this
table, showing more information (like the current leader and term),
hence the generic name.
Adding the table requires some plumbing of
`sharded<raft_group_registry>&` through function parameters to make it
accessible from `register_virtual_tables`, but it's mostly
straightforward.
Also added some APIs to `raft_group_registry` to list all groups and
find a given group (returning `nullptr` if one isn't found, not throwing
an exception).
a483915c62