Dumps CdbExplain_NodeSummary struct in EXPLAIN ANALYZE

`set gp_enable_explain_node_summary=on;`

`drop table if exists tt; create table tt (a int, b int) distributed randomly;`

`explain (analyze,verbose) insert into tt select * from generate_series(1,1000)a,generate_series(1,1000)b;`

`explain (analyze,verbose) select * from tt where a > b;`

```
                                                                                                               QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
 Gather Motion 3:1  (slice1; segments: 3)  (cost=0.00..431.00 rows=1 width=8) (actual time=33.612..78.159 rows=1000 loops=1)
   Output: a, b
   Node Summary:                  vmax       vsum       vcnt       imax
     ntuples:                     1000       1000          1         -1
     execmemused:                    0          0          0          0
     workmemused:                    0          0          0          0
     workmemwanted:                  0          0          0          0
     totalWorkfileCreated:           0          0          0          0
     peakMemBalance:             44592      44592          1         -1
     totalPartTableScanned:          0          0          0          0
     segindex0: -1
     ninst: 1
     StatInsts:
       (segN) pstype starttime counter firsttuple startup total ntuples nloops execmemused workmemused workmemwanted workfileCreated firststart peakMemBalance numPartScanned sortMethod sortSpaceType sortSpaceUsed bnotes enotes
       (seg-1) 241 0.00 0.03 0.08 1000.00 1.00 0 0 0 0 0 0 44592.00 0 0 0 0 0 0 0
   ->  Seq Scan on public.tt  (cost=0.00..431.00 rows=1 width=8) (actual time=2.512..77.640 rows=353 loops=1)
         Output: a, b
         Filter: (tt.a = 100)
         Node Summary:                  vmax       vsum       vcnt       imax
           ntuples:                      353       1000          3          0
           execmemused:                    0          0          0          0
           workmemused:                    0          0          0          0
           workmemwanted:                  0          0          0          0
           totalWorkfileCreated:           0          0          0          0
           peakMemBalance:             19072      57216          3          0
           totalPartTableScanned:          0          0          0          0
           segindex0: 0
           ninst: 3
           StatInsts:
             (segN) pstype starttime counter firsttuple startup total ntuples nloops execmemused workmemused workmemwanted workfileCreated firststart peakMemBalance numPartScanned sortMethod sortSpaceType sortSpaceUsed bnotes enotes
             (seg0) 211 0.00 0.00 0.08 353.00 1.00 0 0 0 0 0 0 19072.00 0 0 0 0 0 0 0
             (seg1) 211 0.00 0.00 0.03 330.00 1.00 0 0 0 0 0 0 19072.00 0 0 0 0 0 0 0
             (seg2) 211 0.00 0.00 0.05 317.00 1.00 0 0 0 0 0 0 19072.00 0 0 0 0 0 0 0
 Planning time: 4.514 ms
   (slice0)    Executor memory: 119K bytes.
   (slice1)    Executor memory: 27K bytes avg x 3 workers, 27K bytes max (seg0).
 Memory used:  128000kB
 Optimizer: Pivotal Optimizer (GPORCA)
 Execution time: 78.584 ms
(39 rows)
```