[SPARK-13245][CORE] Call shuffleMetrics methods only in one thread for ShuffleBlockFetcherIterator

Call shuffleMetrics's incRemoteBytesRead and incRemoteBlocksFetched when polling FetchResult from `results` so as to always use shuffleMetrics in one thread.

Also fix a race condition that could cause memory leak.

Author: Shixiong Zhu <shixiong@databricks.com>

Closes #11138 from zsxwing/SPARK-13245.