Preflight check for usable space and cache size

[todvora]

This PR is adding a datanode preflight check comparing the usable disk space and the setting of node search cache. Recently, there were several complains that datanodes are running out of space and it was not clear why. This preflight check will either terminate the startup if the cache consumes all of the usable space or warn if the cache is over 80% of the space.

With this, users should get better overview and an action recommendation in case the datanode is running on very limited storage and the cache could cause problems with watermark levels inside opensearch.

How Has This Been Tested?

Added unit tests, existing set of integration tests for valid startups.

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.