middletier, itself separated in interface, SQLite, objectstore and
shared tests.
Moved all utilities (exceptions, threading...) to a shared utility
directory.
Created a single, shared unit test from all the scattered ones.

License comment is no longer filtered out by doxygen when generating web pages of the individual source files.

Intermediate commit: started to implement the garbage collection, plus a producer and consumer action.
The Agent class is becomming too complex, so started to split it up between the current agent (living with the application it represents)
and the AgentVisitor, which will look at it from the outside (for the garbage collector).

Some legacy utilities from castor were imported alongside, with some adaptations.