Make DirectoryImplementor#deleteFile apply changes asynchronously

Looks like the deleteFile operation is the main responsible component for the latency generated by writing to the Lucene Directory, but this operation could be processed asynchronously by a background thread.

Configurtion API org.infinispan.lucene.directory.BuildContext could allow for an optional Executor to be passed for this purpose, and if there isn't we keep current behaviour for running it synchronously.

The tricky part I guess is making sure that the tests, which verify written consistency -including delete operations - are refactored to be able to deal with the fact that delete operations will happen eventually.