Got rid of mapCurrentlyFlushing in MemTable (for now let's just be non-threadsafe; we'll worry about concurrency later).  renamed FileIndex.getClosestPositionBeforeOrAtKey() to FileIndex.getStartingIndexInFileForScan(), and made that method be able to handle files whose start key is > the start key of the scan. implemented FileSysTable.get() when there are multiple potential files with the key. changed FileIndex.getFilesPossiblyContainingKeyRange() to return a PriorityQueue with the latest timestamp first.