LUCENE-5904: add tests for segment/gen inflation, fix gen inflation (it always parsed gen of 0), fix off-by-one in gen-inflation (was starting at 2 instead of 1), add robustness to trash files we might be looking at, add minor restriction to segment suffix names so we can always parse generations correctly from them

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/lucene5904@1620779 13f79535-47bb-0310-9956-ffa450edef68