[MINOR] add the "ignore-persist" option to conditionally ignore persistence

[MINOR] add the "ignore-persist" option to conditionally ignore persistence

This is used to disable persistence depending on some conditions (for
example using an ACL matching static files or a specific User-Agent).
You can see it as a complement to "force-persist".

In the configuration file, the force-persist/ignore-persist declaration
order define the rules priority.

Used with the "appsesion" keyword, it can also help reducing memory usage,
as the session won't be hashed the persistence is ignored.