MAINT-5232: Make LLSingleton's 'initializing' stack coro-specific.

The stack we maintain of which LLSingletons are currently initializing only
makes sense when associated with a particular C++ call stack. But each
coroutine introduces another C++ call stack!

Move the initializing stack from function-static storage to
LLSingletonBase::MasterList. Make it a map keyed by llcoro::id. Each coro then
has a stack of its own.

This introduces more dependencies on the MasterList singleton, requiring
additional LLSingleton_manage_master workarounds.