[CALCITE-4129] Support deep equality check for RelNode

[hsyuan]

Currently the only way to check rel node tree deep equality is transforming
into String by RelOptUtil.toString(rel) with
SqlExplainLevel.EXPPLAN_ATTRIBUTES, which is inefficient. One example is
RexSubQuery. It has to do it this way, because the rel being reference by
RexSubQuery is possibly not yet registered to VolcanoPlanner, and the digest
equals checks the input RelNode by identity (not content). That is OK for
RelSubset and HepRelVertex, if the RelNode is already registered in planner,
but not for plain RelNode that is outside of planner. Due to this, we have to
implement another set of deep equals logic in our system.

With this patch, digestEquals() and digestHash() are extended to support
deep equality check for RelNode.

CALCITE-4129