CAMEL-16374: Allow multiple completions for route

[Karen Lease <ka...@gmail.com>]

Hi Camel team,
I'm working on CAMEL-16374. Following the feedback from Claus on my 
first attempt: "The model already allows to add multiple OnCompletion, 
so we could also look at using the model as it is,", I've reworked the 
changes.

My question is:
Shall we allow any number of onCompletion definitions for a route, in 
the same way that it's currently possible for global onCompletions?
This is shown in the test case org.apache.camel.processor. 
OnCompletionMoreGlobalRouteCompletionTest and all of the relevant 
onCompletion routes are invoked.

Since these are inherited by the route if there are no onCompletions 
defined for it, it would seem to make sense to also allow any number on 
the route as well.

If that sounds OK to the group, I'm wrap up my changes and create a pull 
request.

Thanks,
Karen Lease