* feat: verify the availability of PoP participants
* simplified and improved logging
* added MQTT-based availability check for PoP participants
* extended MQTT mocking
* Only the block proposer will send out the MQTT messages to the pop participants
* Added a configuration value for the MQTT response timeout
* removed parallel execution of one test case
* added r/w locking to the MQTT response processing
* set MQTT timeout unit to ms
Signed-off-by: Julian Strobl <jmastr@mailbox.org>
