config: Don't allow storages to share a path

Previously we allowed multiple storage names to use the same path for convenient sharing of Gitaly configs between nodes, but with v17.0 we need to make a breaking change to prevent this.

As part of the write ahead log, partition managers take exclusive ownership of the storage's database, any other storage using that path will fail.

Make non-unique storage paths a configuration error.

Related to #5598 (closed)