Returns block hash by the given height.

URL: ```http:://127.0.0.1:36941/json_rpc```
### Request: 
```json
{
  "id": 0,
  "jsonrpc": "2.0",
  "method": "on_getblockhash",
  "params": [0]
}
```
### Request description: 
```

```
### Response: 
```json
{
  "id": 0,
  "jsonrpc": "2.0",
  "result": ""
}
```
### Response description: 
```

```
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