Refactor how messages are added to mailboxes

DeliverMessage() is now MessageAdd(), and it takes a Mailbox object that it
modifies but doesn't write to the database (the caller must do it, and plenty
of times can do it more efficiently by doing it once for multiple messages).
The new AddOpts let the caller influence how many checks and how much of the
work MessageAdd() does. The zero-value AddOpts enable all checks and all the
work, but callers can take responsibility of some of the checks/work if it can
do it more efficiently itself.

This simplifies the code in most places, and makes it more efficient. The
checks to update per-mailbox keywords is a bit simpler too now.

We are also more careful to close the junk filter without saving it in case of
errors.

Still part of more upcoming changes.

store/train.go

@@ -50,6 +50,18 @@ func (a *Account) OpenJunkFilter(ctx context.Context, log mlog.Log) (*junk.Filte
 	return f, jf, err
 }
 
+func (a *Account) ensureJunkFilter(ctx context.Context, log mlog.Log, jfOpt *junk.Filter) (jf *junk.Filter, opened bool, err error) {
+	if jfOpt != nil {
+		return jfOpt, false, nil
+	}
+
+	jf, _, err = a.OpenJunkFilter(ctx, log)
+	if err != nil {
+		return nil, false, fmt.Errorf("open junk filter: %v", err)
+	}
+	return jf, true, nil
+}
+
 // RetrainMessages (un)trains messages, if relevant given their flags. Updates
 // m.TrainedJunk after retraining.
 func (a *Account) RetrainMessages(ctx context.Context, log mlog.Log, tx *bstore.Tx, msgs []Message) (rerr error) {
@@ -75,11 +87,11 @@ func (a *Account) RetrainMessages(ctx context.Context, log mlog.Log, tx *bstore.
 				return fmt.Errorf("open junk filter: %v", err)
 			}
 			defer func() {
-				if jf != nil {
-					err := jf.Close()
-					if rerr == nil {
-						rerr = err
-					}
+				if rerr != nil {
+					err := jf.CloseDiscard()
+					log.Check(err, "close junk filter without saving")
+				} else {
+					rerr = jf.Close()
 				}
 			}()
 		}