MAINT-5232: Ensure that llcoro::get_id() returns distinct values.

Until now, the "main coroutine" (the initial context) of each thread left
LLCoros::Current() NULL. The trouble with that is that llcoro::get_id()
returns that CoroData* as an opaque token, and we want distinct values for
every stack in the process. That would not be true if the "main coroutine" on
thread A returned the same value (NULL) as the "main coroutine" on thread B,
and so forth. Give each thread's "main coroutine" a dummy heap CoroData
instance of its own.