Enable optimizer_use_gpdb_allocators guc by default and fix resgroup test (#8780)

This guc makes ORCA use gpdb allocators. This allows for faster
optimization due to less overhead, reduced memory during optimization
due to smaller/fewer headers, and makes ORCA observe vmem limits instead
of crashing.

With setting optimizer_use_gpdb_allocators, we now track orca's memory
usage within resource groups/vmem. As a result, we need to update the test as
we go OOM slightly earlier when ORCA is enabled.
Authored-by: NChris Hajas <chajas@pivotal.io>

src/backend/utils/misc/guc_gp.c

@@ -2799,7 +2799,7 @@ struct config_bool ConfigureNamesBool_gp[] =
 			GUC_NO_SHOW_ALL | GUC_NOT_IN_SAMPLE
 		},
 		&optimizer_use_gpdb_allocators,
-		false,
+		true,
 		NULL, NULL, NULL
 	},
 

src/test/isolation2/output/resgroup/resgroup_bypass_optimizer.source

@@ -183,19 +183,16 @@ SELECT * FROM memory_result;
  0           
 (1 row)
 61: SELECT * FROM eat_memory_on_qd;
- hold_memory 
--------------
- 0           
-(1 row)
+ERROR:  Out of memory
+DETAIL:  Resource group memory limit reached
 SELECT * FROM memory_result;
  rsgname        | ismaster | avg_mem 
 ----------------+----------+---------
  rg_bypass_test | 0        | 0.0     
- rg_bypass_test | 1        | 24.0    
+ rg_bypass_test | 1        | 0.0     
 (2 rows)
 61: SELECT * FROM eat_memory_on_qd;
-ERROR:  Out of memory
-DETAIL:  Resource group memory limit reached
+ERROR:  current transaction is aborted, commands ignored until end of transaction block
 61: SELECT * FROM eat_memory_on_qd;
 ERROR:  current transaction is aborted, commands ignored until end of transaction block
 SELECT * FROM memory_result;
@@ -212,7 +209,7 @@ SELECT * FROM memory_result;
  rsgname        | ismaster | avg_mem 
 ----------------+----------+---------
  rg_bypass_test | 0        | 0.0     
- rg_bypass_test | 1        | 24.0    
+ rg_bypass_test | 1        | 20.0    
 (2 rows)
 61q: ... <quitting>