Log errors for snapshot handler
**Describe The Problem:**
Since the latest release of Homebridge, I'm getting these errors in the logs:

```
[Living Room Camera] Snapshot request handler provided empty image buffer!
[Living Room Camera] The image snapshot handler for the given accessory is slow to respond! See https://git.io/JtMGR for more info.
[Living Room Camera] Snapshot request handler provided empty image buffer!
[Living Room Camera] The image snapshot handler for the given accessory is slow to respond! See https://git.io/JtMGR for more info.
[Living Room Camera] Snapshot request handler provided empty image buffer!
[Hall Camera] Snapshot request handler provided empty image buffer!
```
I have no idea what the empty image buffer issue is about, but — if you don't mind me commenting on this – the "slow to respond" issue usually means that you're handling the snapshot requests synchronously when you should be handling them asynchronously. Homebridge polls its accessories in a single thread, and so doesn't like to be kept waiting. It's better just to return a cached snapshot straight away without blocking, and at the same time fire off a function call to get a new snapshot. When the new one arrives, you can call updateCharacteristic() asynchronously.

**To Reproduce:**
Run homebridge-ffmpeg under Hombridge v1.3.1.

**Homebridge Config:**

```json
{
    "name": "Camera ffmpeg",
    "cameras": [
        {
            "name": "Courtyard Camera",
            "videoConfig": {
                "source": "-rtsp_transport tcp -re -i rtsp://admin:Secure99@192.168.185.10",
                "stillImageSource": "-i http://admin:Secure99@192.168.185.10/cgi-bin/snapshot.cgi",
                "maxStreams": 2,
                "maxFPS": 20,
                "maxBitrate": 300,
                "vcodec": "libx264",
                "packetSize": 1316,
                "audio": false,
                "debug": false
            }
        },
        {
            "name": "Garden Camera",
            "manufacturer": "Amcrest",
            "videoConfig": {
                "source": "-rtsp_transport tcp -re -i rtsp://admin:Secure99@192.168.185.11",
                "stillImageSource": "-i http://admin:Secure99@192.168.185.11/cgi-bin/snapshot.cgi",
                "maxStreams": 2,
                "maxFPS": 20,
                "maxBitrate": 300,
                "vcodec": "libx264",
                "packetSize": 1316,
                "audio": false,
                "debug": false
            }
        },
        {
            "name": "Greenhouse Camera",
            "videoConfig": {
                "source": "-rtsp_transport tcp -re -i rtsp://admin:Secure99@192.168.185.12",
                "stillImageSource": "-i http://admin:Secure99@192.168.185.12/cgi-bin/snapshot.cgi",
                "maxStreams": 2,
                "maxFPS": 20,
                "maxBitrate": 300,
                "vcodec": "libx264",
                "packetSize": 1316,
                "audio": false,
                "debug": false
            }
        },
        {
            "name": "Patio Camera",
            "videoConfig": {
                "source": "-rtsp_transport tcp -re -i rtsp://admin:Secure99@192.168.185.13",
                "stillImageSource": "-i http://admin:Secure99@192.168.185.13/cgi-bin/snapshot.cgi",
                "maxStreams": 2,
                "maxFPS": 20,
                "maxBitrate": 300,
                "vcodec": "libx264",
                "packetSize": 1316,
                "audio": false,
                "debug": false
            }
        },
        {
            "name": "Hall Camera",
            "videoConfig": {
                "source": "-rtsp_transport tcp -re -i rtsp://admin:Secure99@192.168.185.14",
                "stillImageSource": "-i http://admin:Secure99@192.168.185.14/cgi-bin/snapshot.cgi",
                "maxStreams": 2,
                "maxFPS": 20,
                "maxBitrate": 300,
                "vcodec": "libx264",
                "packetSize": 1316,
                "audio": false,
                "debug": false
            }
        },
        {
            "name": "Living Room Camera",
            "videoConfig": {
                "source": "-rtsp_transport tcp -re -i rtsp://admin:Secure99@192.168.185.15",
                "stillImageSource": "-i http://admin:Secure99@192.168.185.15/cgi-bin/snapshot.cgi",
                "maxStreams": 2,
                "maxFPS": 20,
                "maxBitrate": 300,
                "vcodec": "libx264",
                "packetSize": 1316,
                "audio": false,
                "debug": false
            }
        }
    ],
    "platform": "Camera-ffmpeg"
}
```

**Environment:**

* **Node.js Version**: v14.15.5
* **NPM Version**: v6.14.11
* **Homebridge Version**: v1.3.1
* **Homebridge Camera FFmpeg Version**: v3.1.0
* **Homebridge Config UI X Plugin Version**: v4.39.1
* **Operating System**: macOS 11.2.2

