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/server.go

@@ -23,3 +23,7 @@ func NewServer(workers *worker.Server, distros *distroregistry.Registry, config
 func (server *Server) V2(path string) http.Handler {
 	return server.v2.Handler(path)
 }
+
+func (server *Server) Shutdown() {
+	server.v2.Shutdown()
+}