Can we allow FSDirectory subclasses to customize whether the ctor does a mkdir? [LUCENE-7375]

Today, just instantiating an `FSDirectory` always brings the directory into existence, even if you will do no writes (`createOutput`).

We have gone back and forth on this, over the ages. E.g. see #1094 (only 2 digits there!![), #1848 (only 3 digits there](), #1848 (only 3 digits there)!), #2538. At one point we created the directory lazily, on the first write (`createOutput`) attempt, but now we always create it when you instantiate `FSDirectory`.

This causes some hassle for consumers, e.g. in https://github.com/elastic/elasticsearch/pull/19338 ES is forking `SimpleFSDirectory` in order to have a read-only directory impl.

Maybe we can do the `Files.createDirectories` in protected method that a subclass could override?

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Migrated from [LUCENE-7375](https://issues.apache.org/jira/browse/LUCENE-7375) by Michael McCandless (@mikemccand)

Start by reading FSDirectory and its SimpleFSDirectory subclass, focusing on the constructor, Files.createDirectories, and createOutput. Trace when directory creation currently occurs and how a read-only subclass such as Elasticsearch's could avoid it. Done means FSDirectory subclasses can customize constructor-time directory creation without breaking the existing write path.

Written by the indexing model from the issue text.