remoteHost := grid.Hosts[1]
	local := grid.Managers[0]

	// 1: Echo
	errFatal(local.RegisterSingleHandler(handlerTest, func(payload []byte) ([]byte, *RemoteErr) {
		t.Log("1: server payload: ", len(payload), "bytes.")
		return append([]byte{}, payload...), nil
	}))
	// 2: Return as error
	errFatal(local.RegisterSingleHandler(handlerTest2, func(payload []byte) ([]byte, *RemoteErr) {
		t.Log("2: server payload: ", len(payload), "bytes.")

In essence, they are `[]byte` -> `[]byte, error` functions.

It is not possible to return *both* an error and a response.

Handlers are registered on the manager using `(*Manager).RegisterSingleHandler(id HandlerID, h SingleHandlerFn, subroute ...string)`.

The server handler function has this signature: `type SingleHandlerFn func(payload []byte) ([]byte, *RemoteErr)`.

Sample handler:
```go

	grid, err := SetupTestGrid(n)
	errFatal(err)
	b.Cleanup(grid.Cleanup)
	// Create n managers.
	for _, remote := range grid.Managers {
		// Register a single handler which echos the payload.
		errFatal(remote.RegisterSingleHandler(handlerTest, func(payload []byte) ([]byte, *RemoteErr) {
			defer PutByteBuffer(payload)
			return append(GetByteBuffer()[:0], payload...), nil
		}))