* feat(mjpeg): add keyframe caching with expiration and cleanup goroutine
* mjpeg: make keyframe cache duration and default usage configurable
* mjpeg: document and add config options for MJPEG snapshot caching
* mjpeg: fix errors after rebase
* Code refactoring for frame.jpeg cache #1155
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Co-authored-by: Alex X <alexey.khit@gmail.com>