session: Use service iterator instead of match/creation callbacks

session: Use service iterator instead of match/creation callbacks

When a new session is created we need to get the initial list
of services which match the session AllowedBearers criteria. As soon
the session is running add or removed services are tracked via the
notifier interface.

We concluded that exposing all the services is not really necessary
and therefore we are using currently two callbacks to get the initial
list. That exposes implementation details from session to service, that
is GSequence. We also need to expose the type 'struct service_entry'.

Instead doing so we are using an iterator and keep all the
implementation details in session.c.