GH-4973 improve performance og inner merge join

[hmottestad]

GitHub issue resolved: #4973

Briefly describe the changes proposed in this PR:

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• my pull request is self-contained
• I've added tests for the changes I made
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[kenwenzel]

I am not sure if something like this could be also helpful in some situations:
https://github.com/linkedfactory/linkedfactory-pod/blob/main/bundles/io.github.linkedfactory.core/src/main/java/io/github/linkedfactory/core/rdf4j/common/query/CompositeBindingSet.java

The binding set is helpful if only few additional bindings are generated via a join. I use this for evaluating joins over time series data.

[hmottestad]

Ahh. So you can keep multiple references to a single binding set and for each of them add another value.

I once made a StatementBindingSet that wrapped a statement and transparently mapped the variable names to the correct component of the statement. That in combination with your suggestion could really make for a much more lightweight binding set.

[hmottestad]

Squash and merge!