Refactor WS clients

[ZdenekM]

Currently, there is one implementation of WS server (used by ARServer and Execution) but there are more implementations of WS clients:

• arcor2_arserver has asyncio client for Execution.
• arcor2_arserver_data has a client for ARServer, that is used in ARServer tests so far.
• arcor2_execution_rest_proxy has a client for Execution.

Ideal state:

• There is a ws module in arcor2 that contains a general implementation of a server (already exists) and a client.
  • The question is whether the general client should be sync or async.
  • It should be possible to run async code from sync one (even within Flask).
  • Another option would be to make it sync and prepare an async wrapper.
• arcor2_arserver_data contains a client for ARServer.
  • To be used in tests, or in some alternative user interfaces.
• arcor2_execution_data contains the client for Execution.
  • To be used by ARServer and Execution Rest Proxy.