dnsdist: Fix memory corruption when using `getAddressInfo`

dnsdist: Fix memory corruption when using `getAddressInfo`

The object holding the callback function, which is translated into
a `LuaContext::LuaFunctionCaller`, needs to be destroyed while holding
the Lua mutex because it will unregister itself from the Lua context,
causing a corruption if a different thread is accessing the Lua context
at the same time.