Rework how CamelServices are added to CamelSession.

* Give CamelServices a simple unique ID string.  That will be its
  identity from now on, not its URL.

* Split adding a CamelService and retrieving a CamelService into two
  separate operations.  Adding a CamelService requires both a UID and
  CamelURL (for now), retrieving a CamelService just requires the UID.

* CamelService now implements the GInitable interface, replacing its
  construct() method.