alternatecalendar: fix race in thread

1. There can be more than one QRunnable running at the same time.
2. s_solarTermsMap in ChineseCalendarProviderPrivate is not guarded by a
   mutex, and it can be used in more than one thread.
3. QCache doesn't clear its cache in FIFO way, so remove QCache and just
   use a plain copy of the result instead, which means only the current
   month is cached, and should be enough in most cases.