Some recipe enhancements
Some recipe enhancements

Tweaked the threaded recipes to use sleep when their pools are starved
which significantly improves efficiency (turns out the main thread was
hogging the pool lock) and added some caveats to the capture while
recording recipe, as suggested by eldonT in #101.

ListFranz/picamera,tfroehlich82/picamera,waveform80/picamera,rocky1001/picamera