Unsolicited replies can cause threading error

If `unsolicited_reply` is called periodically in a thread e.g.:
```python
def get_data_unsolicited(self):
timer = threading.Timer(0.1, self.get_data_unsolicited)
timer.start
self.handler.unsolicited_reply(b"TEST")
```
there will be an occasional stack trace of:
```
Exception in thread Thread-911:
Traceback (most recent call last):
File "c:\instrument\apps\python\lib\threading.py", line 801, in __bootstrap_inner
self.run()
File "c:\instrument\apps\python\lib\threading.py", line 1072, in run
self.function(*self.args, **self.kwargs)
File "C:\Instrument\Apps\epics\support\DeviceEmulator\master\lewis_emulators\mecfrf\interfaces\stream_interface.py", line 48, in get_data_unsolicited
handler.unsolicited_reply(b"TEST")
File "c:\instrument\apps\python\lib\site-packages\lewis\adapters\stream.py", line 116, in unsolicited_reply
self.push(b(reply + self._target.out_terminator))
File "c:\instrument\apps\python\lib\asynchat.py", line 193, in push
self.initiate_send()
File "c:\instrument\apps\python\lib\asynchat.py", line 251, in initiate_send
del self.producer_fifo[0]
IndexError: deque index out of range
```
This stack trace does not appear to affect whether the message is actually sent or not.