[SPARK-8430] ExternalShuffleBlockResolver of shuffle service should support UnsafeShuffleManager

andrewor14 can you take a look?thanks

Author: Lianhui Wang <lianhuiwang09@gmail.com>

Closes #6873 from lianhuiwang/SPARK-8430 and squashes the following commits:

51c47ca [Lianhui Wang] update andrewor's comments
2b27b19 [Lianhui Wang] support UnsafeShuffleManager