Extended `group0_command` to enable transmission of `raft::broadcast_tables::query`.
Added `add_entry_unguarded` method in `raft_group0_client` for dispatching raft
commands without `group0_guard`.
Queries on group0_kv_store are executed in `group_0_state_machine::apply`,
but for now don't return results. They don't use previous state id, so they will
block concurrent schema changes, but these changes won't block queries.
In this version snapshots are ignored.
db88525774