Add a new FileCache class to wrap RepoGroup::findFile() and findFiles(), and make...

Add a new FileCache class to wrap RepoGroup::findFile() and findFiles(), and make wfFindFile() use it by default.  This should improve performance for pages that refer to the same image several times, but the real benefit is that it allows batch file existence checks, à la LinkBatch, by collecting a set of titles (or DB keys) and calling FileCache::findFiles() on them to prefill the cache.
XXX: The code seems to more or less work, but it obviously needs more testing, regarding both stability and memory usage.  In particular, I have not tested file uploads yet -- there may be consistency issues there.