Wait till datanode becomes available during startup

[todvora]

Similarly as with elastic/opensearch, graylog server now waits for an available datanode. Number of attempts and their delay is configurable, reusing elasticsearch_version_probe_attempts and elasticsearch_version_probe_delay, which will be most likely renamed in 6.0.

Motivation and Context

Without the waiting logic, users need to start datanode first and then graylog server, otherwise the server immediately crashes with exception.

How Has This Been Tested?

Existing set of unit tests, manually tested starts

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (non-breaking change)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.

[moesterheld]

Looks good to me. Thank you.

Only one small comment: Should we add a note to misc/graylog.conf that the two properties are also used for the data node connectivity check?