gorsepoll: Don't record items before the last update time.
gorsepoll: Don't record items before the last update time.

This makes adding a new feed behave a little nicer as we no longer
record items from before the time we start tracking the feed.

It also means if feed URIs/IDs change that we don't include lots of
items from the past.

horgh/gorse,horgh/gorse,horgh/gorse