Add an infinite stream of delivery missions

• allow missions not to have an on-disk file; in this case the mission must provide regenerate attribute with a string to execute on load
• make sure save/load serialize/evaluate regenerate
• allow mail messages not to have on-disk files
• implement generation of inside-Tana-colony delivery missions
• implement a timer that generates a new inside-Tana-colony delivery mission whenever you run out of generated delivery missions