lib: have default clock snapshot in event message

lib: have default clock snapshot in event message

The event message is the only message not having a default clock
snapshot property (it's in an event object). Since an event object is
unique and belongs solely to an event message, make it more consistent
and move the default clock snapshot property to the message instead.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>