r/configuration_manager: fixed order in which state is deleted

Previously when group configuration manager persistent state was deleted
we removed configuration map first. If node would crash after the
configuration map was deleted but before deleting the highest known
offset configuration would never be recovered from the log since raft
uses highest known offset as a hint when recovering state during
bootstrap. Fixed problem by removing the highest known offset first,
this way when partial remove will happen raft will still be able to
recover configurations from the log.

Signed-off-by: Michal Maslanka <michal@vectorized.io>
(cherry picked from commit 7f76c89)

src/v/raft/configuration_manager.cc

@@ -413,15 +413,16 @@ ss::future<offset_configuration> configuration_manager::wait_for_change(
 
 ss::future<> configuration_manager::remove_persistent_state() {
     return _storage.kvs()
-      .remove(storage::kvstore::key_space::consensus, configurations_map_key())
+      .remove(
+        storage::kvstore::key_space::consensus, highest_known_offset_key())
       .then([this] {
           return _storage.kvs().remove(
-            storage::kvstore::key_space::consensus, highest_known_offset_key());
+            storage::kvstore::key_space::consensus,
+            next_configuration_idx_key());
       })
       .then([this] {
           return _storage.kvs().remove(
-            storage::kvstore::key_space::consensus,
-            next_configuration_idx_key());
+            storage::kvstore::key_space::consensus, configurations_map_key());
       });
 }