rtcontrol: Add query optimizer

[pyroscope]

• Prefilter the result within rTorrent based on given conditions
• Activated via config option
• See also pyroscope/rtorrent-ps#57

[pyroscope]

:checkered_flag: Already works for some fields, needs rT-PS 1.1+.

10-fold improvement with about 1k items and a small result set, and the more you have, the better it gets.

[chros73]

This is freaking cool! Thank You!!!!!!!!!!! :)