Revert "Fixed StringInstance::getV(...) so it doesn’t use the member _returnValue. Several threads acceding to this function could cause a crash."

This reverts commit 07e50a127b054d58f24e3f36300bb5c249fea4fb.
std::string::c_str() is deallocated when the string is destroyed!
also, be careful to not commit stuff that has nothing to do with the
log message (Support/Plugins/invert.cpp)
partially revert commit cbfc2331b5ff05a712ceeca108cbd81bb26061c2

- revert to original indentation (please be careful)
- StringInstance::getV should not be used: only include it of
OFX_MT_UNSAFE is defined