Allow memory address relations to be cleared
Create thread infrastructure

These changes are a step towards C++11 thread support:

  http://en.cppreference.com/w/cpp/thread

Changes:

* Add ThisThread namespace to allow forward declarations (cf. recorder.h)
* Replace recorder_ptr() extern function by ThisThread::recorder()
* Delete Recorder copy constructor but add move constructor (cf. above)
* Add Threads singleton helper class to start and stop thread recordings
* Add Threads::expect(std::unique_ptr<ReadInstr<bool>>) for assertions
* Add Thread class that encapsulates a static Z3 object