Add 'persist' support to undo 'temporary'

- Added persist_path which reverses the functionality of
  expire_path, thus making a stored record permanent again.
- Added persist to the RedCache::Collector that performs
  persist_path on a path and clears out internal data structures.
Add options to disable 'update' callback

- Added possibility to disable the 'update' callback
  for Collector#register, mainly to be used with
  timeout, to periodically recalculate data.