test skip encoding

[max-hoffman]

prototype before/after:

BenchmarkOltpIndexScan-12    	     206	   5190640 ns/op	 5243491 B/op	  140229 allocs/op
BenchmarkOltpIndexScan-12    	     650	   1785686 ns/op	 3883499 B/op	   20767 allocs/op

The premise of this is a custom row type:

type ProllyRow struct {
	key, value val.Tuple
	kd, vd     val.TupleDesc
	ords       []int // placement access
}

func (r ProllyRow) GetBytes(i int, typ sql.Type) ([]byte, error) {
	pos := r.ords[i]

	if pos < r.kd.Count() {
		ret := r.kd.GetField(pos, r.key)
		return ret, nil
	}

	pos -= r.kd.Count()
	ret := r.vd.GetField(pos, r.value)
	return ret, nil
}

The idea is to go from val.Tuple straight to wire values:

	outVals := make([]sqltypes.Value, len(sch))
	var err error
	if br, ok := row.(sql.BytesRow); ok {
		for i, col := range sch {
			buf, err := br.GetBytes(i, col.Type)
			outVals[i] = sqltypes.MakeTrusted(col.Type.Type(), buf)
			if err != nil {
				return nil, err
			}
		}
		return outVals, nil
	}

• correctness considerations for when storage encoding is not consistent with MySQL byte encoding
• certain values will have to be materialized to satisfy engine logic that operates on Go types
• the ordinal mapping above is 1-step, while main has 2-steps
• this doesn't work with virtual columns