Prevent spurious warnings about thread ids not being correct for messages that are expunged but not yet erased.

Erasing a message (removing the message file from the file system) was made a
separate step a few days ago. The verifydata command checks for consistency of
the data, but didn't correctly skip checking expunged-but-not-yet-erased
messages, leading to the warning.

A similar consistency check in store/account.go does check for that.

I was warned by my nightly backup+veridata script.

verifydata.go

@@ -310,7 +310,7 @@ possibly making them potentially no longer readable by the previous version.
 				}
 				for i, pid := range m.ThreadParentIDs {
 					am := store.Message{ID: pid}
-					if err := db.Get(ctxbg, &am); err == bstore.ErrAbsent {
+					if err := db.Get(ctxbg, &am); err == bstore.ErrAbsent || err == nil && am.Expunged {
 						continue
 					} else if err != nil {
 						return fmt.Errorf("get ancestor message: %v", err)