Cron trigger : first occurrence date and number of occurrences

It should be possible to be more specific when specifiing cron triggers by defining the date of the first occurence and the number of occurence.

Usecases:

Differing the beginning of the execution of a cron trigger to future date.
Limiting the number of occurrences of a cron trigger.
Executing workflows only once at a given date.

Implementation:

* We need to add a column to the cron_triggers_v2 database:
 - remaining_executions

* We need to add two new optionals parameters to the trigger creation:

 -first_execution_first_execution_time: which, if specified, supersedes to the first calculation of "next_execution_time".
 -remaining_occurrences: which, if specified, defines the number of occurrences after which the trigger should be deleted.

* We also need to extend the client to support these new parameters.
The proposed format for the first_execution_time is "YYYY-MM-DD HH:MM" and will be parsed to a datetime (and checked to be in the future).