Excessive CPU usage on Raspberry Pi
Homebridge increased CPU usage significantly after installing this plugin.

### Expected Behavior
CPU usage should stay as it was before installing the plugin (less than 10%), or increase it only slightly.

### Current Behavior
Homebridge running on Raspberry Pi Zero WH with Raspbian Stretch has a ~60% CPU consumption on average.

### Steps to Reproduce (for bugs)
Run Homebridge on Raspberry Pi Zero WH with Raspbian Stretch and Node v10.6.0.

### Versions
- **homebridge**: `0.4.45`
- **homebridge-mi-hygrothermograph**: `1.9.0`
- **node**:       `10.6.0`
- **OS**:         Raspbian 9 (Stretch) on `armv6l`

### Configuration
<!-- If relevant, include the contents of your config.json file between the two ``` lines below.
  - Remove any sensitive information, passwords, etc.
  - If output is long:
    - Please extract only the relevant portions if possible
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```json
  "accessories": [
    {
      "accessory": "Hygrotermograph",
      "name": "Hygrotermograph 1",
      "address": "4C:65:A8:DA:50:90"
    },
    {
      "accessory": "Hygrotermograph",
      "name": "Hygrotermograph 2",
      "address": "4C:65:A8:DA:57:0D"
    }
  ]
```

### Homebridge Log / Command Output
<img width="608" alt="image" src="https://user-images.githubusercontent.com/1714234/51944240-d0f5d700-241b-11e9-8f8d-b129ff2ca226.png">

