Implement feature of new FE/BE protocol whereby RowDescription identifies

Implement feature of new FE/BE protocol whereby RowDescription identifies
the column by table OID and column number, if it's a simple column
reference.  Along the way, get rid of reskey/reskeyop fields in Resdoms.
Turns out that representation was not convenient for either the planner
or the executor; we can make the planner deliver exactly what the
executor wants with no more effort.
initdb forced due to change in stored rule representation.