storage_service: Fix is_starting API

Query _operation_mode on CPU 0.

$ curl -X GET --header "Accept: application/json"
"http://127.0.0.1:10000/storage_service/is_starting"

api/storage_service.cc

@@ -325,8 +325,10 @@ void set_storage_service(http_context& ctx, routes& r) {
         });
     });
 
-    ss::is_starting.set(r, [](const_req req) {
-        return service::get_local_storage_service().is_starting();
+    ss::is_starting.set(r, [](std::unique_ptr<request> req) {
+        return service::get_local_storage_service().is_starting().then([] (auto starting) {
+            return make_ready_future<json::json_return_type>(starting);
+        });
     });
 
     ss::get_drain_progress.set(r, [](std::unique_ptr<request> req) {

service/storage_service.cc

@@ -1180,8 +1180,11 @@ future<sstring> storage_service::get_operation_mode() {
     });
 }
 
-bool storage_service::is_starting() {
-    return _operation_mode == storage_service::mode::STARTING;
+future<bool> storage_service::is_starting() {
+    return smp::submit_to(0, [] {
+        auto mode = get_local_storage_service()._operation_mode;
+        return mode == storage_service::mode::STARTING;
+    });
 }
 
 future<bool> storage_service::is_gossip_running() {

service/storage_service.hh

@@ -2448,7 +2448,7 @@ public:
 #endif
     future<sstring> get_operation_mode();
 
-    bool is_starting();
+    future<bool> is_starting();
 #if 0
     public String getDrainProgress()
     {