CASSANDRA-21093 rework custom configurable startup checks

[smiklosovic]

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The Cassandra Jira

[nickbar01234]

Originally interpreted Map<StartupCheckType, Map<String, Object>> as a mapping from type to a map of checks + options. I'm ok with this approach, so defer to you as the maintainer of Cassandra

[nickbar01234]

What's the purpose of this line?

[smiklosovic]

from javadoc:

If the specified key is not already associated with a value (or is mapped to null) associates it with the given value and returns null, else returns the current value.

This covers cases when there is "enabled" as a key in a map but its value is null. We do not want that. If it is ever null then we want to explicitly put there false.