Moved AICurlPrivate::Stats to AICurlInterface::Stats and added several
counters to keep track of the number of existing instances of
respectively AICurlEasyRequest, AICurlEasyRequestStateMachine,
BufferedCurlEasyRequest, ResponderBase and
ThreadSafeBufferedCurlEasyRequest.
Every curl transaction is a AICurlEasyRequestStateMachine which has a
AICurlEasyRequest as member, which is a reference counting pointer to
a ThreadSafeBufferedCurlEasyRequest. And now BufferedCurlEasyRequest is
derived from CurlEasyRequest which is derived from CurlEasyHandle, but
neither are used separatedly.
