Support matching against multiple keys in ets_lru tables

This change allows the ddoc_cache to appropraitely match against the
objects of the set using new `ets_lru:match_object` and `ets_lru:match`
functions. While it's similar to the `ets:match*` functions, it follows
the convention of returning the value only rather than the stored tuple
as seen in the `ets_lru:lookup` function.