Improvment in opcode cache clear

Of course the "opcache_reset + opcache.fast_shutdown=1" which cause segfault is a PHP bug (https://bugs.php.net/67687) and have to be fixed there.

So 3974de2 is mostly a workaround to this issue, but it's also a improvement as resetting "all" the cache is bad for perf. (Of course most admin will have to reset it after code update, but only once, and atomic deployment is another problem)