Custom expirations using boost "tiers"

This is a custom extension for boost. It is against an older version of boost, so does not match the latest stable, but maybe of use for ideas.

What it does is break down expiration in 2 tiers. One tier expires according to the settings in boost, and other "older" tier is kept in the cache for longer. Older "pager" are consider old. Nodes are kept for longer, since they expire when comments are posted, ...etc.

I had to duplicate some functions as is from boost. This is suboptimal, but perhaps future boost version will provide a better way so as not to do this.

Again, this is here just for ideas.