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.
Contributing
============
Guidelines for developers
-------------------------
- Each module should be contained in a folder of its name (e.g.\
`mod_ping/mod_ping.lua`)
- Each module should have a `README.markdown` page in their folder
with a description, usage, configuration and todo sections (feel
free to copy an existing one as a template)
- Commit messages should begin with the name of the plugin they are
for (e.g. `mod_ping: Set correct namespace on pongs`)
Instructions on cloning the repository are at
<https://prosody.im/doc/installing_modules#prosody-modules>