MINOR: lua-thread: Replace global gL var with an array of states

MINOR: lua-thread: Replace global gL var with an array of states

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

The array of states is initialized at the max number of thread +1.
We define the index 0 is the common state shared by all threads
and should be locked. Other index index are dedicated to each
one thread. The old gL now becomes hlua_states[0].