Add database schema management using Alembic

This patch replaces contrib/realtime/ with a new setup for managing the
database schema required for database integration with Asterisk.  In
addition to initializing a database with the proper schema, alembic can do a
database migration to assist with upgrading Asterisk in the future.
Hopefully this helps make setting up and operating Asterisk with a database
easier.

With this the schema only needs to be maintained in one place instead of
once per database.  The schemas I have added here have a bit of improvement
over the examples that were there before (some added consistency and added
some missing indexes).  Managing the schema in one place here also applies
to all databases supported by SQLAlchemy.

See contrib/ast-db-manage/README.md for more details.

Review: https://reviewboard.asterisk.org/r/2731

patch by Russell Bryant (license 6300)



git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/12@397874 65c4cc65-6c06-0410-ace0-fbb531ad65f3

contrib/realtime/mysql/voicemail_messages.sql

@@ -1,31 +0,0 @@
--- While this does not use the realtime backend, for brevity, we include this table here, as well.
-DROP TABLE IF EXISTS voicemail_messages;
-CREATE TABLE voicemail_messages (
-	-- Logical directory
-	dir CHAR(255),
-	-- Message number within the logical directory
-	msgnum INT(4),
-	-- Dialplan context
-	context CHAR(80),
-	-- Dialplan context, if Voicemail was invoked from a macro
-	macrocontext CHAR(80),
-	-- CallerID, when the message was left
-	callerid CHAR(80),
-	-- Date when the message was left, in Unixtime
-	origtime INT(11),
-	-- Length of the message, in seconds
-	duration INT(11),
-	-- The recording itself
-	recording BLOB,
-	-- Text flags indicating urgency of the message
-	flag CHAR(30),
-	-- Value of channel variable VM_CATEGORY, if set
-	category CHAR(30),
-	-- Owner of the mailbox
-	mailboxuser CHAR(30),
-	-- Context of the owner of the mailbox
-	mailboxcontext CHAR(30),
-	-- Unique ID of the message,
-	msg_id char(40),
-	PRIMARY KEY (dir, msgnum)
-);