[daemon support] hashrate loss due to duplicate jobs received by workers from xmrig-proxy
Multiple instances of `xmrig::DaemonClient` may propogate exactly the same `xmrig::Job`.

Python script to reproduce error with xmrig-proxy that's listening on `127.0.0.1:3333` and connected to `daemon+http://127.0.0.1:18081`
```python
import asyncio
loop = asyncio.new_event_loop()
asyncio.set_event_loop(loop)
import json
login_json = {
  'id': 1,
  'jsonrpc': '2.0',
  'method': 'login',
  'params': {
    'login': '00' * 32,
    'pass': 'x',
    'agent': '',
    'algo': ['rx/0']
    }
  }
login_json_buf = json.dumps(login_json) + '\n'
async def get_job():
    try:
        reader, writer = await asyncio.open_connection(host = '127.0.0.1', port = 3333)
        writer.write(login_json_buf.encode())
        await writer.drain()
        job_json_buf = await  reader.readline()
    except OSError:
        return None
    except asyncio.IncompleteReadError:
        return None
    try:
        writer.close()
        writer._protocol.connection_lost(None)
    except OSError:
        pass
    try:
      job_json = json.loads(job_json_buf)
      print(job_json)
    except json.JSONDecodeError:
      return None
    return job_json
g = asyncio.gather(*[get_job() for i in range(256 * 4)])
r = loop.run_until_complete(g)
received_jobs = sorted([e['result']['job']['blob'] for e in r if e != None], key = lambda e: [e[:79 ], e[79 + 8:], e[79:][:8]] )
uniq_jobs = set(received_jobs)
print({'uniq_jobs': len(uniq_jobs), 'received_jobs': len(received_jobs)})
```