New MutableGraphIndex and ImmutableGraphIndex interfaces

[marianotepper]

The main intent of this PR is to add an interface in front of the OnHeapGraphIndex. This is because it has become clear recently that we will need new implementations for mutable graphs (that are more conservative in the use of heap memory). This PR paves the way towards that path.

Importantly, MutableGraphIndex is protected inside its package, because modifications should only be done through the GraphIndexBuilder class and never directly.

These changes triggered a lot of minor changes everywhere.

[github-actions]

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[marianotepper]

@sam-herman @michaeljmarshall Please make sure that the new design does not preclude any usages of JVector

[sam-herman]

I think this should work well with #519

[michaeljmarshall]

Overall, looks good. I pulled the PR branch to my machine, built it, imported it into CC main (datastax/cassandra@748018e), fixed compilation errors, and ran VectorUpdateDeleteTest and didn't have any test failures.

I left some minor comments mostly on naming/javadocs and one comment about the getView() method that will impact CC.

[tlwillke]

LGTM

[tlwillke]

LGTM

[tlwillke]

Thanks for the fix.