cloudapi: prevent dangling manifest goroutines

When composer exits, it doesn't wait for the manifest generation goroutines
to finish. This is generally a bad practice so let's introduce a bit of
syncing and a new Shutdown method to prevent this.

This also prevents the manifest generation goroutine from creating weird
states when interrupted on a random line of code.

Signed-off-by: Ondřej Budai <ondrej@budai.cz>

internal/cloudapi/v2/v2_test.go

@@ -39,6 +39,7 @@ func newV2Server(t *testing.T, dir string, depsolveChannels []string, enableJWT
 	}
 	v2Server := v2.NewServer(workerServer, distros, config)
 	require.NotNil(t, v2Server)
+	t.Cleanup(v2Server.Shutdown)
 
 	// start a routine which just completes depsolve jobs
 	depsolveContext, cancel := context.WithCancel(context.Background())