[SPARK-4777][CORE] Some block memory after unrollSafely not count into used memory(memoryStore.entrys or unrollMemory)

Some memory not count into memory used by memoryStore or unrollMemory.
Thread A after unrollsafely memory, it will release 40MB unrollMemory(40MB will used by other threads). then ThreadA wait get accountingLock to tryToPut blockA(30MB). before Thread A get accountingLock, blockA memory size is not counting into unrollMemory or memoryStore.currentMemory.
IIUC, freeMemory should minus that block memory

So, put this release memory into pending, and release it in tryToPut before ensureSpace