CLEANUP: lua: fix confusing local variable naming in hlua_txn_new()

CLEANUP: lua: fix confusing local variable naming in hlua_txn_new()

Struct hlua_txn is called "htxn" or "ht" everywhere, while here it's
called "hs" which is the name used everywhere for struct "hlua_smp".
Such confusion participate to the dangers of copy-pasting code, so
let's fix the name here.