Implement mixed index aggregations

[To-om]

Fixes #3202

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• :white_check_mark: login: To-om / name: toom (8fa21ec51012eec7f844af6ba659256a9b81e973)

[analytically]

@To-om

janusgraph-solr/src/main/java/org/janusgraph/diskstorage/solr/SolrIndex.java#L781 Added line #L781 was not covered by tests

[porunov]

@To-om could you please squash your commits into a single commit? If it's complicated then you can just hard reset from this branch: https://github.com/porunov/janusgraph/tree/aggregations Alternatively you can click Allow edits and access to secrets by maintainers on this PR and I will force push into your branch.