Allow handlers to provide validation callables

In order to reject messages before they are processed
make it possible to provide a handler pair that will
validate the incoming message and either reject or allow
the message to be passed to the provided handler.

Part of blueprint wbe-message-validation

Change-Id: Ibd6ee40020c6b98283f40d5bd59e8950d63b7f71
Merge "Allow handlers to provide validation callables"