Add option to expire filter's cache based on access time

[sebaes]

LRU, Soft and Weak are the current FilterCache options, but none of them expire elements based on last access time.

Add expire flag accepting a time value based setting (5s, 10m) that will cause filters to be expired after that time if not accessed.

With current implementations, in theory, the cache would grow to it's maximum if we keep creating filters used not permanently, but for a long enough time to consider caching them. An example would be time based filters, like {today, yesterday, past week, past month}, that are created anew each day.

[kimchy]

Add option to expire filter's cache based on access time, closed by e759b4c.