Enable using existing asyncio loop

[dhoomakethu]

Versions

• Python:
• OS:
• Pymodbus: dev branch
• Modbus Hardware (if used): N/A

Pymodbus Specific

• Client: tcp/rtu/ascii -async

Description

Reported by @makuser in #246 (comment)

TBH I'm not quite sure how to implement this, since I'm using already using my own asyncio loop in my application.

Since It's using loop.run_until_complete in pymodbus/client/async/factory/tcp.py#L97, it would start a new loop on its own, which is not desirable and leads to a Runtime Exception. IMO it should give the option to attach to a loop that's already running.

Code and Logs

 Task exception was never retrieved
future: <Task finished coro=<BAS.async_start() done, defined at /srv/bas/lib/python3.5/site-packages/bas/core.py:70> exception=RuntimeError('This event loop is already running',)>
Traceback (most recent call last):
  File "/usr/lib/python3.5/asyncio/tasks.py", line 239, in _step
    result = coro.send(None)
  File "/srv/bas/lib/python3.5/site-packages/bas/core.py", line 72, in async_start
    await neuron.switch_to_async(self.loop, self.basConfig.alias_dict)
  File "/srv/bas/lib/python3.5/site-packages/bas/brain.py", line 141, in createClient
    loopi, clienti = ModbusClient(schedulers.ASYNC_IO, host=self.modbus_server, port=self.modbus_port, loop=self.loop)
  File "/srv/bas/lib/python3.5/site-packages/pymodbus/client/async/tcp.py", line 45, in __new__
    timeout=timeout, **kwargs)
  File "/srv/bas/lib/python3.5/site-packages/pymodbus/client/async/factory/tcp.py", line 97, in async_io_factory
    client = loop.run_until_complete(asyncio.gather(cor))[0]
  File "/usr/lib/python3.5/asyncio/base_events.py", line 454, in run_until_complete
    self.run_forever()
  File "/usr/lib/python3.5/asyncio/base_events.py", line 408, in run_forever
    raise RuntimeError('This event loop is already running')
RuntimeError: This event loop is already running

[dhoomakethu]

Workarounds :
#246 (comment)

Suggestions:
#246 (comment)