Unexpected message truncation in WaitForAllTransactionsToFinish

Hi,

In WaitForAllTransactionsToFinish():

```
char activity[64];
int rc;

/* Oldest running xid is older than us, so wait */
snprintf(activity,
sizeof(activity),
"Waiting for current transactions to finish (waiting for %u)",
waitforxid);
pgstat_report_activity(STATE_RUNNING, activity);
```

The buffer is not big enough to hold the message if the 'waitforxid' is too big.

How about changing the message to this:

"Waiting for transactions older than %u to finish"

--
Regards,
ChangAo Chen