Zabbix

...G...... [DEV-1147] code review fixes

* commit '2bce8ccfd54ca2027b2ee76fc50d83bac77864f9':
  .......... [DEV-1147] don't register new client if it has no checks to monitor
  ...G...... [DEV-1147] added debug in case of unused exporter is removed
  ...G...... [DEV-1147] style fix suggestion
  ...G...... [DEV-1147] fixed timeout handling
  ...G...... [DEV-1147] removed unsafe logging in thread
  ...G...... [DEV-1147] fixed log
  .......... [DEV-1147] fixed duplicate itemid handling, improved logging
  ...G...... [DEV-1147] renamed queue to indicate that it is plugin queue
  ...G...... [DEV-1147] improved debug logging
  .......... [DEV-1147] updated comments
  .......... [DEV-1147] changed single metric checks to return error instead of empty result
  .......... [DEV-1147] remove inactive clients
  ...G...... [DEV-1147] style fix, use constant instead of magic number
  ...G...... [DEV-1147] removed extra new line
  ...G...... [DEV-1147] removed double call to deactivate in case of invalid update interval
  ...G...... [DEV-1147] removed redundant search in exporter for direct request; replaced all itemid checks in favor to client id checks to determine direct requests for consistency with cleanup
  ...G...... [DEV-1147] use same variable name when accessing client; renamed plugins to pluginsInfo in order to distunguish between plugin and information
  .......... [DEV-1147] fixed handling of item key changes