Skip execution of interruptible mails when yielding

When operators are yielding, for example waiting for async state access to complete before a checkpoint, it would be beneficial to not execute interruptible mails. Otherwise continuation mail for firing timers would be continuously re-enqeueed. To achieve that MailboxExecutor must be aware which mails are interruptible.

The easiest way to achieve this is to set MIN_PRIORITY for interruptible mails.