IMP-1227: Ignore columns of unsupported types in compute stats. Enclose identifiers that are Impala keywords in quotes.

Change-Id: Ie7fa6da2869090428c9229c44b973ecccbb49e8e
Reviewed-on: http://gerrit.ent.cloudera.com:8080/1357
Reviewed-by: Alex Behm <alex.behm@cloudera.com>
Tested-by: jenkins