This introduces a helper to build a query part comparing a field against a past datetime (determined by '$now - $some_interval'), eliminating certain boilerplate code.
Prior to this, a filter test could needlessly result in up to 100 backend requests (limit 100, max_offset 10000) when the filter's associated feeds+categories have fewer than 10000 entries.
This causes the size of the array to be incorrectly doubled due to the original regex match items being combined with the custom items (i.e. the ones with just 'url' and 'size' keys).
Also rework 'RSSUtils::encode_srcset()' a bit so it looks similar to 'RSSUtils::decode_srcset()'.
* add configurable log level for update daemon (DAEMON_LOG_LEVEL)
* when daemon log level is set to LOG_EXTENDED (2) log queries for feed
update selection
