main, view: Pair view builder drain with its start

Before these changes, we only drained the view builder when stopping it,
which was paired with its *construction*. However, since the view
builder depends on group0_service, which is initialized AFTER the view
builder is constructed, that could lead to segmentation faults.
To amend it, we pair draining the view builder with its start, while
leaving the lambda destroying it paired with its construction. This way
we should have a proper initialization order, avoid bugs, and reinforce
our understanding of how Scylla starts up.

Fixes #20772

db/view/view.cc

@@ -2097,7 +2097,7 @@ future<> view_builder::drain() {
 
 future<> view_builder::stop() {
     vlogger.info("Stopping view builder");
-    return drain();
+    co_return;
 }
 
 view_builder::build_step& view_builder::get_or_create_build_step(table_id base_id) {

main.cc

@@ -2190,6 +2190,9 @@ To start the scylla server proper, simply invoke as: scylla server (or just scyl
             if (cfg->view_building()) {
                 view_builder.invoke_on_all(&db::view::view_builder::start, std::ref(mm), utils::cross_shard_barrier()).get();
             }
+            auto drain_view_builder = defer_verbose_shutdown("draining view builders", [&] {
+                view_builder.invoke_on_all(&db::view::view_builder::drain).get();
+            });
 
             api::set_server_view_builder(ctx, view_builder).get();
             auto stop_vb_api = defer_verbose_shutdown("view builder API", [&ctx] {

test/lib/cql_test_env.cc

@@ -1028,6 +1028,9 @@ class single_node_cql_env : public cql_test_env {
             _view_builder.invoke_on_all([this] (db::view::view_builder& vb) {
                 return vb.start(_mm.local());
             }).get();
+            auto drain_view_builder = defer([this] {
+                _view_builder.invoke_on_all(&db::view::view_builder::drain).get();
+            });
 
             // Create the testing user.
             try {