Reimplement DUChainReferenceCounting: QMap => array
I've been running KDevelop built on the first commit in this branch (but with maxIntervalCount = 2 and QT_FORCE_ASSERTS defined in referencecounting.cpp) for five days and haven't experienced any crashes or assertion failures.
All kdevelop and kdev-python tests still pass (with DUChainReferenceCountingEnabler too). None of the 5 KDevelop benchmarks detected any performance impact of the three commits in this merge request.
See the commit messages for details.