Use versioned index directories for repository indices.

Use versioned index directories for repository indices.

Change from the index version of a repository index being stored in a config
file to also using index directories with the version in the name. For that,
`LuceneRepoIndexStore` is added, which adds the fixed `lucene` part to the path.
It also gives out the location of the `lucene.conf` file, which is now stored in
the index directory. This way it is automatically deleted when the directory is
deleted.

I believe that it should also provide means to store branch aliases and tips,
i.e. hide the config file completely. But this isn't implemented with this
commit, the `LuceneService` is still aware that a config file is used.