Automatically add causation id

[codeliner]

Detect dispatch sessions automatically by using causation id and nesting level

Each call to $eventMachine->on() should add a nesting level.
Each call to $eventMachine->dispatch() should remove a nesting level.

First call to $eventMachine->dispatch() should use message.uuid as causation id.

Nesting level + causation id should be set in message metadata.