[SPARK-9482] [SQL] Fix thread-safey issue of using UnsafeProjection in join

This PR also change to use `def` instead of `lazy val` for UnsafeProjection, because it's not thread safe.

TODO: cleanup the debug code once the flaky test passed 100 times.

Author: Davies Liu <davies@databricks.com>

Closes #7940 from davies/semijoin and squashes the following commits:

93baac7 [Davies Liu] fix outerjoin
5c40ded [Davies Liu] address comments
aa3de46 [Davies Liu] Merge branch 'master' of github.com:apache/spark into semijoin
7590a25 [Davies Liu] Merge branch 'master' of github.com:apache/spark into semijoin
2d4085b [Davies Liu] use def for resultProjection
0833407 [Davies Liu] Merge branch 'semijoin' of github.com:davies/spark into semijoin
e0d8c71 [Davies Liu] use lazy val
6a59e8f [Davies Liu] Update HashedRelation.scala
0fdacaf [Davies Liu] fix broadcast and thread-safety of UnsafeProjection
2fc3ef6 [Davies Liu] reproduce failure in semijoin