AgentManager: avoid recursion agentTypeAdded->readAgentTypes->agentTypeAdded

and the same with agent instances.
With 20 instances, this would make the dbus call agentInstances()
20 times, when getting the notification that akonadi is starting up
(so all processes are doing this at the same time, very bad for performance).

New solution: read agent types+instances at startup if Akonadi::Control is online,
otherwise when getting the notification that it is, or when getting the
signal that a type/instance was added. Always emit signals, but of course
when doing this from the constructor nobody is listening (no problem, fast).
The old code was using a different code path to avoid emitting signals for
the initially loaded types+instances, so the behavior is the same.

REVIEW: 124603
(cherry picked from commit 6c26346c)