SPARK-2634: Change MapOutputTrackerWorker.mapStatuses to ConcurrentHashMap

MapOutputTrackerWorker.mapStatuses is used concurrently, it should be thread-safe. This bug has already been fixed in #1328. Nevertheless, considering #1328 won't be merged soon, I send this trivial fix and hope this issue can be solved soon.

Author: zsxwing <zsxwing@gmail.com>

Closes #1541 from zsxwing/SPARK-2634 and squashes the following commits:

d450053 [zsxwing] SPARK-2634: Change MapOutputTrackerWorker.mapStatuses to ConcurrentHashMap