Delay messages for same receiver

[manumoreira]

instedd/surveda#753

[bcardiff]

Should this be a nuntium wide behavior or should it be scoped by channel or app?

[matiasgarciaisaia]

It should probably apply to every SMS-ish channel (ie, not sure if I'll do it for emails/Twitter - if those still work).

The delay should be between messages from the same channel.