Fix WatchedItemStore last-seen stashing logic

Fix WatchedItemStore last-seen stashing logic

This should be the "last revision seen" timestamp, which is
different than the "first revision not seen" timestamp in the DB.

Also make sure that SpecialWatchlist accounts for the stash values.

Relatedly, better document the callback usage in BagOStuff::merge().

Change-Id: I98b03a5cd40fec5b4a2633d499ff77079d264e3c