Remove 'latest' indexes on posts & headers.

Both have an index for the last 100 revisions already - the exact same index can
be used to fetch the most recent change.
I assume the reason this was not yet the case, is that the history-index returns
up to 100 ids, which are then all fetched. However, by passing along $options,
we can make sure rowCompacter doesn't go and fetch more than needed.

Advantage of deleting this separate index is that both history & latest-post
will now use the same index to fetch revisions. LocalBufferedStorage will save
that result, so that saves 1 memcached request where both history & most recent
revision are needed.

Change-Id: Idd2acf6896a335e652789a368b31fbf365892493
Merge "Remove 'latest' indexes on posts & headers."