in "mox uidbumpvalidity", bump to the next uidvalidity, otherwise we likely leave the uidvalidities in inconsistent state

the inconsistent state isn't really harmful, but we don't want inconsistencies.

pointed out in issue #61 by x8x

doc.go

@@ -807,7 +807,7 @@ Change the IMAP UID validity of the mailbox, causing IMAP clients to refetch mes
 This can be useful after manually repairing metadata about the account/mailbox.
 
 Opens account database file directly. Ensure mox does not have the account
-+open, or is not running.
+open, or is not running.
 
 	usage: mox bumpuidvalidity account [mailbox]
 

main.go

@@ -2102,7 +2102,7 @@ func cmdBumpUIDValidity(c *cmd) {
 This can be useful after manually repairing metadata about the account/mailbox.
 
 Opens account database file directly. Ensure mox does not have the account
-+open, or is not running.
+open, or is not running.
 `
 	args := c.Parse()
 	if len(args) != 1 && len(args) != 2 {
@@ -2118,8 +2118,12 @@ Opens account database file directly. Ensure mox does not have the account
 		}
 	}()
 
-	var uidvalidity uint32
 	err = a.DB.Write(context.Background(), func(tx *bstore.Tx) error {
+		uidvalidity, err := a.NextUIDValidity(tx)
+		if err != nil {
+			return fmt.Errorf("assigning next uid validity: %v", err)
+		}
+
 		q := bstore.QueryTx[store.Mailbox](tx)
 		if len(args) == 2 {
 			q.FilterEqual("Name", args[1])
@@ -2132,8 +2136,7 @@ Opens account database file directly. Ensure mox does not have the account
 			return fmt.Errorf("looking up mailbox %q, found %d mailboxes", args[1], len(mbl))
 		}
 		for _, mb := range mbl {
-			mb.UIDValidity++
-			uidvalidity = mb.UIDValidity
+			mb.UIDValidity = uidvalidity
 			err = tx.Update(&mb)
 			if err != nil {
 				return fmt.Errorf("updating uid validity for mailbox: %v", err)