IMPALA-1030: HdfsTableSink was evaluating exprs in Prepare()

Exprs need to be prepared and opened before calling GetValue().

Change-Id: I51d111b79c3453c9ab7acad14b93566f03decbcc
Reviewed-on: http://gerrit.ent.cloudera.com:8080/2959
Reviewed-by: Skye Wanderman-Milne <skye@cloudera.com>
Tested-by: jenkins