Adding a Cache Invalidation strategy

[thom4parisot]

As received feedback, using WP_DEBUG is not a good strategy on live servers, as we may need an always rebuild cache without being in debug.

These rules are interesting:

1. On file expiry;
2. Only for logged in users;
3. For every pages.

Either to choose that from a constant + bitmask, or many constants.

[thom4parisot]

Problem is addressed by Automatic Compilation

[thom4parisot]

We're done with that. No need to do more complicated at the moment.