Use R__LOCKGUARD2 instead of R__LOCKGUARD

To avoid having the first lock order incorrect because the Cint lock
is not yet created when PluginManager first calls R__LOCKGUARD, I
switched to R__LOCKGUARD2 which will create that Cint lock if ROOT
has decided that locks should be made but hasn't yet made them.
Merge pull request #2940 from Dr15Jones/takeRootCintLockBeforeLoadingPlugins

Multithreading fixes -- Use R__LOCKGUARD2 instead of R__LOCKGUARD