getfeeds api: only select the columns we need for performance
db scheme: improve performance
API add endpoint: support multiuser
API endpoint getfeeds: vastly improve performance

The old version used LEFT JOINs with feed_items, which caused MySQL to generate e.g. for a feed with 2K items at 2Kbyte row length, 4MB of rows and so, over 280MB temp tables.
Additional slowdown was created by unneccessary grouping and filtering.

Now, the query uses dependent subqueries, which can entirely run with the indices and don't need temp tables on the disk.