distsql: minor cleanup of processor add code in planner
distsql: minor cleanup of processor add code in planner

Adding a helper function that adds a processor and returns the index.

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