cache eviction

[rakslice]

Users will want a way to avoid wasting space with unwanted cache content, typically unnecessary old versions of things

Alternatives:

• implement something automatic like an age limit or oldest-first size limit
• add command line options for one shot runs that remove specified stuff from the cache
  • need to make sure that removed files don't reappear through syncs
  • maybe save removals and propagate them through syncs