Prepare for XPC

[sfionov]

XPC is an Apple proprietary RPC on top of Mach ports. It is written in C and more lightweight than NSDistantObject (it is on Mach ports too), and even more than eDistantObject.

However, it requires asynchronous interface for exported objects.

We should make callbacks onNewConnection and onCertificateEvent asynchronous with completion methods (handlers?).

Upd: Seems that XPC has synchronous proxies via [NSXPCConnection remoteObjectProxyWithErrorHandler:], so we don't really need noving to async methods