Support DROP STATS <table name>

Adds support for dropping all table and column stats from a table. Once incremental
stats are supported, this will provide the user a way to force a recompute of all
stats.

Change-Id: I27e03d5986b64eb91852bfc3417ffa971d432d6b
Reviewed-on: http://gerrit.ent.cloudera.com:8080/3533
Reviewed-by: Lenni Kuff <lskuff@cloudera.com>
Tested-by: jenkins