Atomic message delivery interface

For SMTP/LMTP-based message delivery, there should be an interface that allows message to be efficiently (and preferrably atomically) stored into a mailbox of multiple accounts.

```go
type Delivery interface {
Rcpt(ctx context.Context, accountName string, folderRole folder.Role) error
Size(bodySize uint64) error
Body(r io.Reader) error
Commit() error
Close() error
}
```