Anchoring co-routines and properly dropping references to allow GC to clean them up

[Chris Gagnon <cgagnon@...>]

Lets assume we have a MainState, and a "ThreadTable" in the global table

lua_getfield(MainState, LUA_GLOBALSINDEX, "ThreadTable");
lua_State *thread = lua_newthread(MainState);
luaL_ref(MainState, -2);

lua_pop(MainState,1);