[SPARK-1888] enhance MEMORY_AND_DISK mode by dropping blocks in parallel

It's unefficient to drop memory blocks to disk inside a synchronized block as IO is slow. As the TODO says, we just need synchronize selecting blocks to be dropped. So my implementation is: in `ensureFreeSpace`, we iterate entries and select blocks to be dropped. But instead of dropping block inside `ensureFreeSpace`, we can just mark selected entries as dropping, and return these blocks, let the caller do the dropping. When other thread call `ensureFreeSpace` again, they will skip entries that marked as dropping when iterating entries. And the caller, `tryToPut`, will do the dropping before put the new block into entries. In this way, we can do dropping in parallel.
And both the two threads comes here, the condition "`maxMemory - (currentMemory - selectedMemory) >= space`" will  be true for both threads. In fact, the memory that dropped is not enough for the two thread, since the two threads both viewed the `freeMemory` as available and used twice.