Core: Terms filter lookup caching should cache values, not filters.

The terms filter lookup mechanism today caches filters. Because of this, the
cache values depend on two things: the values that can be found in the lookup
index AND the mapping of the local index, since changing the mapping can change
the way that the filter is parsed. We should make the cache depend solely on
the content of the lookup index.

For instance the issue I was seeing was due to the following scenario:
 - create index1 with _id indexed
 - run terms filter with caching, the parsed filter looks like `_id: 1 OR _id: 2`
 - remove index1
 - create index1 with _id not indexed
 - run terms filter without caching, the parsed filter is `_uid: type#1 OR _uid: type#2` (the _id field mapper knows how to use the _uid field when _id is not indexed)
 - run terms filter with caching, the filter is fetched from the cache: `_id: 1 OR _id: 2` but does not match anything since `_id` is not indexed.