Send full updates after old last seen updates.

Send full updates after old last seen updates.

Previously, whenever we saw that there was a previous update that a
client would have seen, we simply calculated the delta set based on
which properties have changed, and would most likely send an
incremental update set (excepting the case of a new or newly sent
announcement, in which case all sent updates are full).

However, if the last seen update was old, and there's a chance that
a user may have run RGS since, it is possible that due to the
7-day-backdating-mechanism included on the client, the reference
update would no longer be present.

To fix that, anytime we see that a last seen update is more than six
days old, we automatically include a full update.