Multi-threaded dense_dot in HashedDocDotFeatures

Made dense_dot() and add_to_dense() of HashedDocDotFeatures work in multi-threaded situations. This happened by adding a virtual get_copy() method to the CTokenizer, that allows to create a copy of the appropriate subclass.
Also, made CHashedDocConverter correctly SG_REF the tokenizer it uses.
Finally, SG_FREE'd an array in CPRCEvaluation class to fix a memory leak.