IMPALA-1055: Compute stats query statements don't quote DB and table names

The compute stats statement was not quoting the DB and table names. If those names
were aliasing with keywords, then the compute stats would not execute due to a syntax
error.

Change-Id: Ie08421246bb54a63a44eaf19d0d835da780b7033
Reviewed-on: http://gerrit.ent.cloudera.com:8080/3170
Reviewed-by: Ippokratis Pandis <ipandis@cloudera.com>
Tested-by: jenkins