Zabbix

A.FG...PS. [ZBXNEXT-3950,ZBXNEXT-6092] added MQTT support for Zabbix agent 2

Merge in ZBX/zabbix from feature/ZBXNEXT-3950-5.1 to master

* commit '5a4ce744d068eddcd56bd885dfbf213b2ac5adf9': (49 commits)
  .D........ [ZBXNEXT-3950,ZBXNEXT-6092] fixed change log
  .D........ [ZBXNEXT-3950] fixed a typo
  .D........ [ZBXNEXT-3950,ZBXNEXT-6092] fixed change log
  .D........ [ZBXNEXT-3950] added change log
  ...G...... [ZBXNEXT-3950] code clean-up and small fix in zabbix agent 2 version print
  .......... [ZBXNEXT-3950] changed items type and value type to constant in api tests
  .......... [ZBXNEXT-3950] added test cases with mqtt.get key for Selenium autotests
  .......... [ZBXNEXT-3950] added test cases with mqtt.get key for api autotest
  A.F....... [ZBXNEXT-6092] fixed code style
  ...G...... [ZBXNEXT-3950] fixed panic on empty mqtt client, and error message for mqtt passive check
  ..F....... [ZBXNEXT-6092] fixed appearance conditions of custom interval field for mqtt.get item key
  ...G...... [ZBXNEXT-3950] fixed invalid reporting of url
  ...G...... [ZBXNEXT-3950] fix issues based on QA sub-tasks
  ...G...... [ZBXNEXT-3950] added MQTT copyright notice to version information
  ...G...... [ZBXNEXT-3950] added description in config for mqtt timeout option
  ...G...... [ZBXNEXT-3950] make item unsupported in case of subscription error
  ...G...... [ZBXNEXT-3950] changed to use configured timeout to avoid long hardcoded timeouts
  ...G...... [ZBXNEXT-3950] improved logs
  ...G...... [ZBXNEXT-3950] fixed possibility of connection being leaked, improved logs
  ...G...... [ZBXNEXT-3950] improved client ID
  ...