Move the variables into a simple new DUChainReferenceCounting class to
avoid the repetition of the `thread_local` keyword.

Reorder the declarations and initializations of two variables -
refCountingHasAdditionalRanges and refCountingRanges - to minimize
sizeof(DUChainReferenceCounting) while preserving the logic of the
variable ordering and grouping within the class.

Eliminate the mutex and the redundant dependency between threads.

This should work perfectly if the reference counting of each object is
confined to a single thread. This will break code that calls
DUChainReferenceCounting functions on the same object from multiple
threads. Hopefully such code does not exist.

This change does not break any kdevelop or kdev-python tests.