[SPARK-4120][SQL] Join of multiple tables with syntax like SELECT .. FROM T1,T2,T3.. does not work in SparkSQL

Right now it works for only 2 tables like below query.
sql("SELECT * FROM records1 as a,records2 as b where a.key=b.key ")

But it does not work for more than 2 tables like below query
sql("SELECT * FROM records1 as a,records2 as b,records3 as c where a.key=b.key and a.key=c.key").

Author: ravipesala <ravindra.pesala@huawei.com>

Closes #2987 from ravipesala/multijoin and squashes the following commits:

429b005 [ravipesala] Support multiple joins