mod_pubsub_feeds: Track latest timestamp seen in feeds instead of last poll This should ensure that an entry that has a publish timestmap after the previously oldest post, but before the time of the last poll check, is published to the node. Previously if an entry would be skipped if it was published at 13:00 with a timestamp of 12:30, where the last poll was at 12:45. For feeds that lack a timestamp, it now looks for the first post that is not published, assuming that the feed is in reverse chronological order, then iterates back up from there.

Over the years, some servers stop working for various reasons, and leave
behind broken roster subscriptions that trigger failing s2s connections.
This module allows cleaning up such cases by unsubscribing local users
from their contacts on those servers. Also works for typos and the like.
Use with care.

```lua
Component "gmail.com" "unsubscriber"
```