Return last N results by sortkey when using sortkey_after

In the SQL index implementation, sortkey_after queries are always
rewritten to use one of the ChangeAccess.*Prev() methods, which order
results by sort key ascending. Update the Lucene implementations to
match this sort order. Specify in the ChangeIndex javadoc that this
sort order is required in such cases, since other code in
QueryProcessor depends on this assumption.

Change-Id: Ie35d2c6bd07963db6d33145a83493d12c6a4289d
4 files changed