[FEATURE] Schedule messages for delivery

[imyousuf]

Is your feature request related to a problem? Please describe.
Instead of instant dispatch we want the ability to schedule a message at a future time. This message should be deleteable before its dispatched. And schedule can be one time or repeated; in case of repeated it can be specified how many times or till when.

Describe the solution you'd like
Introduce a new concept of "ScheduledMessage" which will have a dispatchDate specified; additionally if cron is specified it has to be accompanied with times or until and not both.

Describe alternatives you've considered
Until this functionality is available clients will have to create their own scheduler for this purpose; but that seems too much and hence we will implement it.