Dispose off-heap memory when MemoryStore is cleared

If buffer in MemoryStore's SerializedMemoryEntry is allocated off-heap, current clear() action doesn't dispose them. These off-heap memory, though, will be freed when process ends, but for application that need to stop and restart SparkContext serveral times, this may cause some kind of off-heap memory leak.
It may be better to dispose off-heap memory by itself when clearing MemoryStore.