Transactional event listeners

The Dynamic Config Brigade came to me with a problem. They need to be able to group primitive events together into a single logical unit to make a decision based on them. This actually seems to me to be a limitation of the current transaction API in ONOS. Transactions allow a set of modifications to be committed atomically, but there's currently no way to associate events that occur atomically with one another. We need to rethink event APIs to some extent to provide a way to associate events that occur atomically within a transaction with one another.