Also organised common in subdirectories, and added sub directory for mockDB.

Moved all of the classes in middletier/common to the scheduler directory because most of them are specific to the scheduler.  Please feel free to move those that are no so specific elsewhere.

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.