Summary of Changes

There are situations where the index of Smart Search can get poluted with old data (unused terms and taxonomies, already deleted content items) and where the only solution so far has been to clear the reindex and start indexing all over again. This is not something that you really want to do in a live scenario with a large site, so this attempts some different solutions.
First of all, it runs a garbage collection, which by default deletes all entries, which aren't present in the base table of the base component. We then delete all orphaned term and taxonomy mappings as well as all then orphaned taxonomies and terms. In the end, we then execute the already previously present optimisation queries. To trigger all of this, a new dropdown button was added in the backend named "Maintenance", where you can choose between purging the index and doing the optimisation.
Note: The garbage collection has been added as a default to the Adapter base class and there could be situations where this results in a fail. I'm open for ideas here.

Testing Instructions

1. Apply the PR
2. Click on "Optimise" in the backend
3. Afterwards see, that searching still works properly.

Notes

1. Since 4.1 is in RC, this will have to go into 4.2. Since there is no 4.2-dev branch yet, I'm pointing this towards 4.1-dev for now.