Unexpected message truncation in WaitForAllTransactionsToFinish

[cca5507]

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