I can't attach diff to valentin's review, so here's a new one.

Optimized monitor queries when there are only filters related to `ia_job` table.

Changed ia_job indexes from (*, submit_time) to (*, id).

When using a large offset mysql queries are very slow even when using index if there are a lot of other fields in the SELECT. When filters are only related to `ia_job` we do a SELECT `id` FROM `ia_job` query which uses the available indexes to return the job ids fast and ordered properly and then do the full query with a WHERE `ID` in (...).

There is a great performance increase for all queries for which we had ia_job indexes but were slowed down by mysql's poor limit handling.

Benchmarks: http://hackers.devnet.ro/wiki/Benchmarks

The monitor?round=arhiva&first_entry=100000 is still slow, but I think this is because round is a varchar(64) and thus the index_key is pretty big. If we add numeric ids to round this should work a lot better too (but that's for another patch).