New recents behavior to match spec.

Updated behavior queries each documents root for recently modified
documents.  It uses a new variant of DirectoryLoader which limits the
maximum number of parallel queries to relieve memory pressure.  When
first started, it waits up to 500ms for everyone to finish, then
publishes whatever results are ready, and then refreshes results as
each straggler finishes.

New RootCursorWrapper that always blends in authority and rootId
columns, which are used for binding root details from blended cursors.

Bug: 10593596, 10329994
Change-Id: Icc0d4a2f1b6166edc72f78a4c88f444eeba6f2f0