Refactoring of L3 agent notifications for router

Refactoring of L3 agent notifications for router

The goal of this refactoring is to reduce duplication by
replacing the L3EventObservers mechanism (a specific
mechanism for L3 agent notifications), with the
CallbacksManager mechanism (a more general mechanism
currently in use), so that there is one method
used.

This is the first part of refactoring the L3 agent so that
it uses the new neutron.callbacks mechanism. To do this,
duplicate calls will be made for notifications related to
the router, only using the new callback mechanism.

This commit does two things. First, it puts in place the
notifiers for the new callback mechanism. Second, it updates
the metatdata proxy agent (which is in the same repo) to
use the new callback mechanism.

Later commits will update other repos from the old to new
callback mechanism, and to then remove the old callback
mechanism, once no longer used.

Change-Id: If134947957fd671aa99a0b2d2b37f7ec65e37766
Partial-Bug: #1433552