Optimisation of load_tables()

Got this email from Andrew Donkin:

> I haven't tested this but looking at the code it seems to me that
> load_tables() is seek()ing far more often that it needs to, if cache
> mode is disabled.
>
> function load_tables() {
> if (is_array($this->cache_translations) && <===========
> is_array($this->table_originals) &&
> is_array($this->table_translations))
> return;
>
> $this->cache_translations is never set if enable_cache is false, so
> load_tables() always performs the seeks and reads to initialise
> table_originals and table_translations.

Above statements are correct, so the only reasoning behind current behaviour might be that it allows MO files to change while application is executing. Not a very convincing reason to keep it, especially since you'll want to disable cache only for very large files where the memory footprint will outweight the benefits of caching.

So, let's fix it.