KAFKA-7502: Cleanup KTable materialization logic in a single place (#6174)

This is a draft cleanup for KAFKA-7502. Here is the details:

* Make KTableKTableJoinNode abstract, and define its child classes ([NonMaterialized,Materialized]KTableKTableJoinNode) instead: now, all materialization-related routines are separated into the other classes.

* KTableKTableJoinNodeBuilder#build now instantiates [NonMaterialized,Materialized]KTableKTableJoinNode classes instead of KTableKTableJoinNode.

Reviewers: Guozhang Wang <wangguoz@gmail.com>,  Bill Bejeck <bbejeck@gmail.com>