Update framework principal on re-registration

From the design doc:

This is a bit involved because ‘principal’ is used for authentication and rate limiting.
The authentication part is straightforward because a framework with updated ‘principal’ should authenticate with the new ‘principal’ before being allowed to re-register. The ‘authenticated’ map already gets updated when the framework disconnects and reconnects, so it is fine.
For rate limiting, Master:failoverFramework() needs to be changed to update the principal in ‘frameworks.principals’ map and also remove the metrics for the old principal if there are no other frameworks with this principal (similar to what we do in Master::removeFramework()).
The Master::visit() and Master::_visit() should work with the current semantics.

NOTE that the principal type used in Mesos has changed since this ticket was originally filed; the master code should be checked thoroughly to ensure that the above is still relevant!