[SPARK-4467] fix elements read count for ExtrenalSorter

the elementsRead variable should be reset to 0 after each spilling

Author: Tianshuo Deng <tdeng@twitter.com>

Closes #3302 from tsdeng/fix_external_sorter_record_count and squashes the following commits:

7b56ca0 [Tianshuo Deng] fix method signature
782c7de [Tianshuo Deng] make elementsRead private, fix comment
bb7ff28 [Tianshuo Deng] update elemetsRead through addElementsRead method
74ca246 [Tianshuo Deng] fix elements read count

(cherry picked from commit d75579d09912cfb1eeac0589d625ea0452701fa0)
Signed-off-by: Andrew Or <andrew@databricks.com>