Ksql: Support stream-stream joins

[apurvam]

Currently, you can only perform stream-table joins (stream: left, table: right). We should support stream-stream joins, too, similar to what Kafka's Streams API already supports.

Related tickets:

• GH-760: Support table-table joins

[IDispose]

table-table joins also should be supported.

[miguno]

@IDispose and @pouledodue : Please cast your +1 vote to the separate table-table join ticket that I just created: https://github.com/confluentinc/ksql/issues/760

[apurvam]

Fixed by #1417