Reduce stream startup time by constantly running FFmpeg
**Is your feature request related to a problem? Please describe:**
I'm always frustrated waiting several seconds for the live stream to start from my cameras. All my cameras and my homebridge server are on a wired ethernet network with plenty of bandwidth.

**Describe the solution you'd like:**
I'd like a per-camera option to permanently establish the FFmpeg connection from homebridge to the camera if possible rather than to start this connection each time a live stream is requested by HomeKit as is currently the case.

**Describe alternatives you've considered:**
I have tried various switches and settings in ffmpeg to reduce the initial latency but none of these seem to have a significant effect.

**Additional context:**
Several HikVision cameras used. I would enable this for at least one or two of my cameras but not necessarily all.

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