[#12293] YSQL: Use better defaults for copy command

Summary:
With AsyncFlush landed in master (D16005), we adjust the default values to achieve a better copy performance. Note that we cannot set ROWS PER TRANSACTION too high as it will OOM the tserver; once packed column is enabled, we will set it at higher value (see D16717).

Note that if one sets ROWS PER TRANSACTION too high, we get the error:

```
ysqlsh:/home/centos/gen_table/insert_script.sql:6: ERROR:  Remote error: Service unavailable (yb/rpc/yb_rpc.cc:165): Call rejected due to memory pressure: Call yb.tserver.PgClientService.Perform 172.151.28.7:54138 => 172.151.28.7:9100 (request call id 2301)
```

Test Plan:
jenkins

Run and test on portal (make sure to disable packed column):
On copying 1M rows with 2 CPUs:
```
512 ysql_session_max_batch_size, 20K rows per transaction:
01:50.504
3072 ysql_session_max_batch_size, 20K rows per transaction:
01:45.250
```

Reviewers: smishra, rthallam

Reviewed By: rthallam

Subscribers: yql

Differential Revision: https://phabricator.dev.yugabyte.com/D16752

src/postgres/src/include/commands/copy.h

@@ -19,7 +19,7 @@
 #include "parser/parse_node.h"
 #include "tcop/dest.h"
 
-#define DEFAULT_BATCH_ROWS_PER_TRANSACTION  1000
+#define DEFAULT_BATCH_ROWS_PER_TRANSACTION  20000
 
 /* CopyStateData is private in commands/copy.c */
 typedef struct CopyStateData *CopyState;

src/yb/yql/pggate/pggate_flags.cc

@@ -53,7 +53,7 @@ DEFINE_uint64(ysql_prefetch_limit, 1024,
 DEFINE_double(ysql_backward_prefetch_scale_factor, 0.0625 /* 1/16th */,
               "Scale factor to reduce ysql_prefetch_limit for backward scan");
 
-DEFINE_uint64(ysql_session_max_batch_size, 512,
+DEFINE_uint64(ysql_session_max_batch_size, 3072,
               "Use session variable ysql_session_max_batch_size instead. "
               "Maximum batch size for buffered writes between PostgreSQL server and YugaByte DocDB "
               "services");