Too many temporary std::string in Mutex and decaf::lang::Integer::toString(int)

Every Mutex will associated with a MutexProperties and the name of the Mutex is actually generated by the DEFAULT_NAME_PREFIX plus a call to Integer::toString( ++id ).

From the profiling of our application (highly multi-threaded), we found that many times are spent in the std::basic_string allocation and deallocation inside the functions decaf::lang::Integer::toString(int) and decaf::lang::Integer::toString(int, int).