Add support for an expiry date tag.

[ramesius]

Originally reported by @klee

Sometimes there are stacks that need to live for longer than 7 days but that we still dont want hanging around for ever. In this case it would be good to be able to tag a slice with the a don't kill before date. t

Given a stack has a dont-kill-before tag
And is older than 7 days but younger than the tag date
When Batman assesses the slice
Then Batman wont kill it.

Given a stack has a dont-kill-before tag
And is older than the tag date
When Batman assesses the slice
Then Batman will kill it.