Major Description
Normally Lucene is write-once (except for segments.gen file, which NRTCachingDir never caches), but in some tests (TestDoc, TestCrash) we can write the same file more than once.
I don't think NRTCachingDir should have these asserts, and I think on createOutput it should remove any old file if present.
I also found & fixed a possible concurrency issue (if more than one thread syncs at the same time; IndexWriter doesn't ever do this today but it has in the past).