馃崚 runners/calculator: implement thread-safety in QalculateEngine::evaluate
Libqalculate does not seem to support ability to run multiple computations that are controlled or have timeout set beeing run in the same time. After the timeout was introduced in QalculateEngine this led to BUG 470219, which happens when computations are started from multiple threads in the same time that "confuses" libqalculate computation thread which leads to crash in libqalculate code.
To fix that we need to ensure that only one evaluation is running at single moment of time. This is done via QalculateLock class that is like QMutexLocker but for libqalculate. QalculateLock is implemented with two static mutexes. Mutex s_evalLock is used to ensure that only one evaluation is running at single moment. Mutex s_ctrlLock is used to ensure that thread that aborted evaluation will get to start next evaluation.
(cherry picked from commit 9d18e082)