Add connected property to AsyncModbusClientMixin.

[d21d3q]

When using pymodbus with pyserial-asyncio ther is problem when calling protocol right after creation pair of transport an protocol.
On transport initialization protocol.connection_made callback is being added via loop.call_soon so that this example works because client.read_coils is called from coroutine (after several event loop iterations).

When I use it like this:

transport, protocol = await create_serial_connection(...)
coils = await protocol.read_coils(...)

I am getting error 'NoneType' object has no attribute 'write' because transport is not assigned yet to protocol.

connected property allows for such workaround:

while not protocol.connected:
    await asyncio.sleep(0)  # let event loop run

Maybe transport should be responsible for awaiting connection_made inside initializer?

[d21d3q]

Another thing that I've included in this PR is checking if future is done before setting exception.
In case of using in in this way:

result = await asyncio.wait_for(client.read_holding_registers(...), timeout=0.5)

When reaching timeout, inner future is being canceled, and I am obtaining InvalidStateError comming from here