alembic: Adjust sippeers, queue_members, and voicemail_messages tables.

* Increased the sippeers useragent max string size to 255.

* Changed the queue_members uniqueid to an auto incremented integer
instead of a string.

* Increased the voicemail_messages BLOB size to LONGBLOB on mysql.

* Fixed the add_tables_for_pjsip config change version downgrade actions
to drop a table it created.

* Adjusted the sample alembic.ini files cdr.ini.sample, config.ini.sample,
and voicemail.ini.sample to give a mysql and postgres sqlalchemy.url
lines.

ASTERISK-23847 #close
Reported by: Stephen More

ASTERISK-23825 #close
Reported by: Stephen More

ASTERISK-23909 #close
Reported by: Stephen More

Review: https://reviewboard.asterisk.org/r/3870/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@420211 f38db490-d61c-443f-a65b-d21fe96a405b