MINOR: lua-thread: Replace state_from by state_id

MINOR: lua-thread: Replace state_from by state_id

The goal is to allow execution of one main lua state per thread.

"state_from" is a pointer to the parent lua state. "state_id"
is the index of the parent state id in the reference lua states
array. "state_id" is better because the lock is a "== 0" test
which is quick than pointer comparison. In other way, the state_id
index could index other things the the Lua state concerned. I
think to the function references.