IMPALA-950: Skip computing stats for decimal columns.

This patch also adds a mechanism to return analysis warnings to
client, which is used to log skipped decimal columns.

Change-Id: I30c246044a68ec8861cd5bed072bd54e65a079e6
Reviewed-on: http://gerrit.ent.cloudera.com:8080/2822
Reviewed-by: Skye Wanderman-Milne <skye@cloudera.com>
Tested-by: jenkins