Set default value of guc log_rotation_size so that log file would be rotated each 1GB. (#7264)

Previously the default value is 0, so normally log rotation happens each
day by default according to the other log related guc configurations.
That often makes a single log file rather large in real users'
environment.  Limiting the log file size so that people could easiler
open and search the log files.
Reviewed-by: NDaniel Gustafsson <dgustafsson@pivotal.io>

src/backend/utils/misc/guc.c

@@ -2344,7 +2344,7 @@ static struct config_int ConfigureNamesInt[] =
 			GUC_UNIT_KB
 		},
 		&Log_RotationSize,
-		0, 0, INT_MAX / 1024,
+		1 * 1024 * 1024, 0, INT_MAX / 1024,
 		NULL, NULL, NULL
 	},