[SPARK-4064]NioBlockTransferService.fetchBlocks may cause spark to hang.

cc @rxin

Author: GuoQiang Li <witgo@qq.com>

Closes #2929 from witgo/SPARK-4064 and squashes the following commits:

20110f2 [GuoQiang Li] Modify the exception msg
3425225 [GuoQiang Li] review commits
2b07e49 [GuoQiang Li] If we create a lot of big broadcast variables, Spark may hang