Callback handling - delivery acknowledgement + status update

A delivery acknowledgment can be monitored via the callback URL.

Code flow:
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The callback file could be: extern/sms.php - [input: carrier=clickatell for example]. This file would be the main handler for processing acknowledgments irrespective of provider.

The callback url could be sth like http://example.com/sites/all/modules/civicrm/extern/sms.php?provider=clickatell

The callback file based on input / provider could ask specific provider files to complete the processing, example - CRM/SMS/Provider/Clickatell.php (if provider=Clickatell)

A base file CRM/SMS/Provider.php parent of CRM/Core/SMS/{$Provider}.php would try to do most of processing by itself. Specific operations only be delegated to CRM/SMS/Provider/{$provider}.php based on the amount of functionality that differs from base class.

Action / post process
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For now activity would be updated with status & appropriate message (in location field).
apiMsgId stored in result column of activity table (by $provider->send() action) would now be used to uniquely identify & update an activity record.

Hooks would allow creating custom tables and updating other params time, cost .. values.
Since $provider->send() does not set the status initially, any kind of status would be enough to indicate that there has been a callback, and status column would always reflect the current status at that point of time.