[cul] Fix 2s blocking problem after sending FS20 message

[MichaelKlemm]

Currently the CUL sending thread will produce a timeout of 2s if a command is sent that will not provide a response. This is the case when sending a FS20 message ("F" prefix). The major purpose of this PR is to provide a generic solution for such commands and only fix the problem for the FS20 protocol as there is no other equipment for testing available.

Additional Minor changes and refactorings:

• Use a queue that is blocking instead of polling in a busy loop
• Use monitors / synchronized methods to wait for a response instead of using a boolean local member

[MichaelKlemm]

Hi! How to proceed with this PR?