IMPALA-995: Add plan hints embedded in comments and preserve them in views.

This patch adds two new hint styles:
1. Traditional commented hint: /* +hint1,hint2,hint3 */
2. End-of-line commented hint: -- +hint1,hint2,hint3\n

We now preserve hints when creating views. We always use the
end-of-line commented hint style to allow Hive to read
hinted views created by Impala. Hive does not support
traditional /* */ comments, and attempts to parse /*+ */ as
hints, failing with a parse error on unrecognized hints.

This patch also changes Impala to only issue a warning
for unrecognized hints instead of throwing an error. This
allows Impala to run against hinted views created by Hive.

Change-Id: I6e8352442e763c0029f72c17363caa087572dca0
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4235
Reviewed-by: Alex Behm <alex.behm@cloudera.com>
Tested-by: jenkins
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4361