Spinoff from Lucene 6.2.0 upgrade: https://github.com/elastic/elasticsearch/pull/20147/files/0ccfe6978918b9a756b3c882613e8e392aafc7e1#r76148215

As of Lucene 6.2.0, Directory.createOutput now requires that the specified file does not already exist, but this caused test failures in the snapshot/restore tests because we do sometimes overwrite a file during restore in some cases.

I think ES should instead prune unused files up front before doing this?