Use StringInfo when logging distributed snapshot info

Under GCC 8, I get a warning (and rumor has it that you get this under
GCC 7 with `-Wrestrict`):

```
sharedsnapshot.c: In function ‘LogDistributedSnapshotInfo’:
sharedsnapshot.c:924:11: warning: passing argument 1 to
restrict-qualified parameter aliases with argument 4 [-Wrestrict]
  snprintf(message, MESSAGE_LEN, "%s, In progress array: {",
           ^~~~~~~
     message);
     ~~~~~~~
sharedsnapshot.c:930:13: warning: passing argument 1 to
restrict-qualified parameter aliases with argument 4 [-Wrestrict]
    snprintf(message, MESSAGE_LEN, "%s, (dx%d)",
             ^~~~~~~
       message, ds->inProgressXidArray[no]);
       ~~~~~~~
sharedsnapshot.c:933:13: warning: passing argument 1 to
restrict-qualified parameter aliases with argument 4 [-Wrestrict]
    snprintf(message, MESSAGE_LEN, "%s (dx%d)",
             ^~~~~~~
       message, ds->inProgressXidArray[no]);
       ~~~~~~~
```

Upon further inspection, the compiler is right: according to C99, it is
undefined behavior to pass aliased arguments to the "str" argument of
`snprint` (`restrict`-qualified function parameters, to be pedantic).

To make this safer and more readable, this patch switches to using the
StringInfo API. This change might come with a teeny tiny bit of
performance because of:

1. stack vs heap allocation
2. larger initial allocation size of StringInfo

But this area of the code is *never* a hot spot, and `appendStringInfo`
and friends are arguably faster than our old call patterns of
`snprintf`, so I won't sweat on that.

(cherry picked from commit 89553ad2)
(Back port of greenplum-db/gpdb#5753)

src/backend/utils/time/sharedsnapshot.c

@@ -896,18 +896,18 @@ AtEOXact_SharedSnapshot(void)
 void
 LogDistributedSnapshotInfo(Snapshot snapshot, const char *prefix)
 {
-	static const int MESSAGE_LEN = 500;
-
 	if (!IsMVCCSnapshot(snapshot))
 		return;
 
+	StringInfoData buf;
+	initStringInfo(&buf);
+
 	DistributedSnapshotWithLocalMapping *mapping =
 		&(snapshot->distribSnapshotWithLocalMapping);
 
 	DistributedSnapshot *ds = &mapping->ds;
 
-	char message[MESSAGE_LEN];
-	snprintf(message, MESSAGE_LEN, "%s Distributed snapshot info: "
+	appendStringInfo(&buf, "%s Distributed snapshot info: "
 			 "xminAllDistributedSnapshots=%d, distribSnapshotId=%d"
 			 ", xmin=%d, xmax=%d, count=%d",
 			 prefix,
@@ -917,18 +917,23 @@ LogDistributedSnapshotInfo(Snapshot snapshot, const char *prefix)
 			 ds->xmax,
 			 ds->count);
 
-	snprintf(message, MESSAGE_LEN, "%s, In progress array: {",
-			 message);
+	appendStringInfoString(&buf, ", In progress array: {");
 
 	for (int no = 0; no < ds->count; no++)
 	{
 		if (no != 0)
-			snprintf(message, MESSAGE_LEN, "%s, (dx%d)",
-					 message, ds->inProgressXidArray[no]);
+		{
+			appendStringInfo(&buf, ", (dx%d)",
+					 ds->inProgressXidArray[no]);
+		}
 		else
-			snprintf(message, MESSAGE_LEN, "%s (dx%d)",
-					 message, ds->inProgressXidArray[no]);
+		{
+			appendStringInfo(&buf, " (dx%d)",
+					 ds->inProgressXidArray[no]);
+		}
 	}
+	appendStringInfoString(&buf, "}");
 
-	elog(LOG, "%s}", message);
+	elog(LOG, "%s", buf.data);
+	pfree(buf.data);
 }