MINOR: lua: Protect the common Lua state during a runtime filter deinit

MINOR: lua: Protect the common Lua state during a runtime filter deinit

hlua_filter_deinit_per_thread() releases the filter reference from the
shared Lua state without taking the global lock. This is fine during the
startup and the shutdown, where it is the only user of the state, but
the proxies dynamically created with "add backend" are destroyed at
runtime with "del backend", with their filter instances finalized: their
deinit callback may then run while other threads execute Lua code on the
shared state. Take the global lock when releasing the reference of the
common state, as all the other runtime users of this state do.