LocalDirAllocator should incorporate LocalStorage

The o.a.h.fs.LocalDirAllocator is not aware of o.a.h.m.t.LocalStorage (introduced in MAPREDUCE-2413) - it always considers the configured local dirs, not just the ones that happen to be good. Therefore if there's a disk failure then every call to get a local path will result in LocalDirAllocator#confChanged doing a disk check of all the configured local dirs. It seems like LocalStorage should be a private class to LocalAllocator so that all users of LocalDirAllocator benefit from the disk failure handling and all the various users of LocalDirAllocator don't have to be modified to handle disk failures. Note that LocalDirAllocator already handles faulty directories.