added some mutexes to IECore.ClassLoader to fix some threading issues we were experiencing
added some mutexes to IECore.ClassLoader to fix some threading issues we were experiencing

appleseedhq/cortex,hradec/cortex,lento/cortex,appleseedhq/cortex,danieldresser/cortex,davidsminor/cortex,danieldresser/cortex,davidsminor/cortex,appleseedhq/cortex,lento/cortex,hradec/cortex,davidsminor/cortex,goddardl/cortex,davidsminor/cortex,lento/cortex,hradec/cortex,goddardl/cortex,danieldresser/cortex,goddardl/cortex,danieldresser/cortex