Use with Locker, possible in a dedicated stack frame, like this:
// ... code ...
{ // open stack frame
	Locker locker(iu->payload());
	iu->payload()["a"] = "string";
	iu->payload()["b"] = "simultaneouslySentString";
} // close stack frame, sending the update

Also renamed some functions (preserving old names as deprecated aliases)

Also changed constructors of PayloadEntryProxy to protected.

Setting default channel name and default payload type works OK

It is Herwin's resend fix. Test sources are present but not
enabled during library build (will have to implement test build asap).