[SPARK-21315][SQL] Skip some spill files when generateIterator(startIndex) in...

[SPARK-21315][SQL] Skip some spill files when generateIterator(startIndex) in ExternalAppendOnlyUnsafeRowArray.

## What changes were proposed in this pull request?

In current code, it is expensive to use `UnboundedFollowingWindowFunctionFrame`, because it is iterating from the start to lower bound every time calling `write` method. When traverse the iterator, it's possible to skip some spilled files thus to save some time.

## How was this patch tested?

Added unit test

Did a small test for benchmark:

Put 2000200 rows into `UnsafeExternalSorter`-- 2 spill files(each contains 1000000 rows) and inMemSorter contains 200 rows.
Move the iterator forward to index=2000001.

*With this change*:
`getIterator(2000001)`, it will cost almost 0ms~1ms;
*Without this change*:
`for(int i=0; i<2000001; i++)geIterator().loadNext()`, it will cost 300ms.

Author: jinxing <jinxing6042@126.com>

Closes #18541 from jinxing64/SPARK-21315.