api: Expose the reservoir frame delay as a settable option

There's no reason to explicitly bind this to the max keyframe interval.
It's useful to, for example, set it larger than 1.5x the keyframe
interval for short intervals (1-3s) to prevent bitrate starvation after
larger than expected (complex) keyframes.

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>