Return current blockchain height

URL: ```http:://127.0.0.1:11211/getheight```
### Request: 
```json
{
}
```
### Request description: 
```

```
### Response: 
```json
{
  "height": 11111,
  "status": "OK"
}
```
### Response description: 
```
  "height": Height of the blockchain (equals to top block's height + 1).
  "status": Status of the call.

```
