Added mutexs in problem

Implementing issue #190 (closed)

Two of the protected attributes (both std::map: state_block_nofitication_map_ and factor_nofitication_map_) had getXXX() functions that returned via reference. They have been changed to consumeXXX() that returns them via std::move so, emptying them after the call.

In the process, I identified that the list Problem::state_block_list_ became useless some time ago. It was removed and all the associated machinery.