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)