Work around warnings reported by GCC with -O3
Fix Helgrind failure with valgrind=1 make flag

Helgrind gets confused when mutexes are stack-allocated.
GCC 4.8 with -O0 got unlucky in its generated code, as false
negatives were reported.
Fix return value of callbacks in test
Test AnswerContext need success callback
Ref counting assertions in tests
Test responding to queue item with no answer
More unit tests for AnswerContext.