topology_state_machine: Introduce lock transition

Will be used in load balancer tests to prevent concurrent topology
operations, in particular background load balancing.

load balancer will be invoked explicitly by the test. Disabling load
balancer in topology is not a solution, because we want the explicit
call to perform the load balancing.

docs/dev/topology-over-raft.md

@@ -124,6 +124,9 @@ Additionally to specific node states, there entire topology can also be in a tra
     it from group 0. We also use this state to rollback a failed bootstrap or decommission.
 - `rollback_to_normal` - the decommission or removenode operation failed. Rollback the operation by
     moving the node we tried to decommission/remove back to the normal state.
+- `lock` - the topology stays in this state until externally changed (to null state), preventing topology
+    requests from starting. Intended to be used in tests which want to prevent internally-triggered topology
+    operations during the test.
 
 When a node bootstraps, we create new tokens for it and a new CDC generation
 and enter the `commit_cdc_generation` state. Once the generation is committed,

service/storage_service.cc

@@ -744,6 +744,8 @@ future<> storage_service::topology_state_load(state_change_hint hint) {
                 return read_new_t::no;
             }
             switch (*state) {
+                case topology::transition_state::lock:
+                    [[fallthrough]];
                 case topology::transition_state::join_group0:
                     [[fallthrough]];
                 case topology::transition_state::tablet_migration:

service/topology_coordinator.cc

@@ -2341,6 +2341,10 @@ class topology_coordinator : public endpoint_lifecycle_subscriber {
             case topology::transition_state::tablet_resize_finalization:
                 co_await handle_tablet_resize_finalization(std::move(guard));
                 break;
+            case topology::transition_state::lock:
+                release_guard(std::move(guard));
+                co_await await_event();
+                break;
             case topology::transition_state::left_token_ring: {
                 auto node = get_node_to_work_on(std::move(guard));
 

service/topology_state_machine.cc

@@ -153,6 +153,7 @@ static std::unordered_map<topology::transition_state, sstring> transition_state_
     {topology::transition_state::left_token_ring, "left token ring"},
     {topology::transition_state::rollback_to_normal, "rollback to normal"},
     {topology::transition_state::truncate_table, "truncate table"},
+    {topology::transition_state::lock, "lock"},
 };
 
 // Allows old deprecated names to be recognized and point to the correct transition.

service/topology_state_machine.hh

@@ -120,6 +120,7 @@ struct topology {
         left_token_ring,
         rollback_to_normal,
         truncate_table,
+        lock,
     };
 
     std::optional<transition_state> tstate;