Change the module-static thread_specific_ptr to a function-static
thread_specific_ptr so it will be initialized on demand -- since LLSingleton
will need to rely on get_id(). Note that since LLCoros isa LLSingleton, we
must take great care to avoid circularity.

Introduce a private helper class LLCoros::Current to obtain and bind that
thread_specific_ptr. Change all existing internal references from the static
thread_specific_ptr to the new Current helper class.