Possibly later things that need to be thread-local can be handled through
the ScInterpreterContext.

Why handle some thread-local things through the
ScDocument::maNonThreaded and ScDocument::maThreadSpecific mechanism,
and others through this ScInterpreterContext? Good question.

Share SvNumberFormatter across worker threads
and use mutex to protect SvNumberFormatter::IsNumberFormat()

Change-Id: I372e5fbd9a19785f55f0faf4a4bedc5fc1ef3e03