MINOR: lua-thread: Use NULL context for main lua state

MINOR: lua-thread: Use NULL context for main lua state

The goal is to no longer use "struct hlua" with global main lua_state.

This patch returns NULL value when some code tries go get the hlua struct
associated with a task through hlua_gethlua(). This functions is useful
only during runtime because the struct hlua contains only runtime states.

Some Lua functions allowed to yield are called from init environment.
I'm not sure this is a good practice. Maybe it will be clever to
disallow calling this kind of functions.