Cron for automated message delivery

Add a new cron api civicrm_api('Job', 'process_sms') which works similar to CRM_Mailing_BAO_Mailing::processQueue but uses sms api instead of sending mailings.

CRM/Mailing/BAO/Job->deliverGroup method will need to be improved to use $sms->send (instead of $mailier->send) based on if a mailing has sms_provider_id set.

We would need to make sure that mailing_recipients and mailing_event_queue tables are populated with phone_id columns, which would help make cron compatible with sms.

Checks will have to be done to make sure process is not stopped in case of missing 'from email account', since for sms mode its sms_provider_id that would be required. There may be other such cases.