New common transport layer.

[janiversen]

Internally both client and server uses async transport, in order to optimise code maintenance.

The transport layer is responsible for handing:

• connect/disconnect
• receive data, with frame handling
• send data, with automatic retry (if configured)
• automatic reconnect (if configured)
• Diagnostic (if configured)

The transport layer is solely internal to pymodbus and do not expose any API to the outside world. States like e.g. connected, are NOT exposed directly. but must be added in the different interfaces.

Note: the transport layer is independent of the actual transport method tcp/udp/tls/serial.

This is the first version, it contains a lot of simplifications:

• clients no longer have different connection_made and connection_lost co-routines.
• transport is integrated in client base.

[alexrudd2]

This introduced a tiny type error (#1511), which was easy enough to solve.

[janiversen]

Thanks, and sorry for that.