a debug tool to detect LLPointer issues for SH-694: check if there are any...

a debug tool to detect LLPointer issues for SH-694: check if there are any other LLPointer issues in the mesh model uploading flow and fix them if exist.
This debug tool is off by default. To turn it on, set LL_REF_COUNT_DEBUG to be 1 in the header file "llcommon/llrefcount.h".