[SPARK-2897][SPARK-2920]TorrentBroadcast does use the serializer class specified in the spark option "spark.serializer"

Author: GuoQiang Li <witgo@qq.com>

Closes #1836 from witgo/SPARK-2897 and squashes the following commits:

23cdc5b [GuoQiang Li] review commit
ada4fba [GuoQiang Li] TorrentBroadcast does not support broadcast compression
fb91792 [GuoQiang Li] org.apache.spark.broadcast.TorrentBroadcast does use the serializer class specified in the spark option "spark.serializer"

(cherry picked from commit ec79063fad44751a6689f5e58d47886babeaecff)
Signed-off-by: Reynold Xin <rxin@apache.org>