sql: reorganize explain_plan.go for clarity.
sql: reorganize explain_plan.go for clarity.

glycerine/cockroach,vaibhavarora/cockroach,glycerine/cockroach,maxlang/cockroach,maxlang/cockroach,paperstreet/cockroach,paperstreet/cockroach,paperstreet/cockroach,vaibhavarora/cockroach,maxlang/cockroach,vaibhavarora/cockroach,glycerine/cockroach,vaibhavarora/cockroach,vaibhavarora/cockroach,maxlang/cockroach,paperstreet/cockroach,glycerine/cockroach,vaibhavarora/cockroach,maxlang/cockroach,maxlang/cockroach,paperstreet/cockroach,vaibhavarora/cockroach,maxlang/cockroach,maxlang/cockroach,vaibhavarora/cockroach,glycerine/cockroach,glycerine/cockroach,paperstreet/cockroach,paperstreet/cockroach,paperstreet/cockroach,glycerine/cockroach,glycerine/cockroach