Solve stop/start problems of some client/server combinations.

[janiversen]

as #1207 points out, there are a problem when stopping a client/server in some combinations.

add a new set of tests (test_server_task.py) that test 3 different scenarios:

• Start server then close (server is in listen mode)
• Start server then Start/read/stop client then stop server
• Start server then Start/read client then stop server and finally stop client
  This is done for all protocols serial/tcp/tls/udp
  and for sync as well as async.

Since this seems to be a timing problem all tests will be run 10 times to provoke intermittent issues.

fixes #1207
fixes #1202
fixes #1198

[janiversen]

Current status:

• there is a strange problem with windows (object deletion fails, due to hanging task)
• async tls have problems especially on v3.8, this is known problems with async itself
• test_examples.py needs to be updated.

[janiversen]

Work continues here: https://github.com/janiversen/pymodbus_nextGen/pull/3