[SPARK-21923][CORE] Avoid calling reserveUnrollMemoryForThisTask for every record

## What changes were proposed in this pull request?
When Spark persist data to Unsafe memory, we call  the method `MemoryStore.putIteratorAsBytes`, which need synchronize the `memoryManager` for every record write. This implementation is not necessary, we can apply for more memory at a time to reduce unnecessary synchronization.

## How was this patch tested?

Test case (with 1 executor 20 core):
```scala
val start = System.currentTimeMillis()
val data = sc.parallelize(0 until Integer.MAX_VALUE, 100)
      .persist(StorageLevel.OFF_HEAP)
      .count()

println(System.currentTimeMillis() - start)

```

Test result:

before

|  27647  |  29108  |  28591  |  28264  |  27232  |

after

|  26868  |  26358  |  27767  |  26653  |  26693  |

Author: Xianyang Liu <xianyang.liu@intel.com>

Closes #19135 from ConeyLiu/memorystore.