MAINT-5232: Add DEBUG logging to LLSingleton dependency tracking.

Specifically, add DEBUG logging to the code that maintains the stack of
LLSingletons currently being initialized. This involves passing
LLSingletonBase's constructor the name of LLSingleton's template parameter
subclass, since during that constructor typeid(*this).name() will only produce
"LLSingletonBase".

Also add logdebugs() and oktolog() helper functions.