Complete the ast_storage project for voicemail

by John Khvatov for The Asterisk Project

The project aims at creating a storage engine with several backends for use in asterisk applications. This engine will allow to write a storage independent code in an application, select a storage backend type at run-time (optionally per-user), add new storage backend without modifying the core application code. Also, as a part of the project, it is planned to rewrite app_voicemail with ast_storage engine.