IMPALA-1284: Allow implicit cross joins

This patch adds support for executing implicit cross joins in Impala. An
implicit cross join occurs when two tables are referenced in the FROM
clause of a select statement without specifying the join type and
in absence of applicable equi-join predicate.

To convert an implicit join into a cross join, we manually create the
cross join node during the join reordering. When two sub-plans are
compared to each other a Hash Join plan is always preferred.

As a side effect, explicit cross joins that have equi join conjuncts are
now rewritten to hash joins.

This patch enables us to run TPC-DS queries Q61 and Q88 which are added
as planner and query tests.

Change-Id: Ifd53a78e8eb38d553eb039bfeef0216e438790ba
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4695
Reviewed-by: Martin Grund <mgrund@cloudera.com>
Tested-by: jenkins