BUG/MINOR: hlua: improper lock usage in hlua_filter_new()

BUG/MINOR: hlua: improper lock usage in hlua_filter_new()

In hlua_filter_new(), after each hlua resume, we systematically try to
empty the stack by calling lua_settop(). However we're doing this without
locking the lua context, so it is unsafe in multithreading context if the
script is loaded using 'lua-load'. To fix the issue, we protect the call
with hlua_{lock,unlock}() helpers.

This should be backported up to 2.6.