XtceDb: add a namespace for each space-system

When looking up NameDescriptions in the XtceDb, you now
always need to know the full name under some namespace.

It is possible to look up:
 - by NamedObjectId
 - by qualifiedName (string)
 - by a combination of the namespace and the name
   within that namespace

Furthermore, the XtceDb implementation was extended to allow
retrieving SpaceSystems in the same manner as Containers and
Parameters.
Add more information to extraction result of container

Previously only the SequenceContainer was returned. With
this commit, we also return the slice (hierarchy) where it the
container starts, and the position of the container within that
slice.
Add ContainerRequestManager

Similar to the ParameterRequestManager, there is now a
ContainerRequestManager that delegates container subscriptions
of multiple PacketConsumers to one XtceTmProcessor.
Merge pull request #20 from fqqb/container-request-manager

Container request manager