Anything that would happen after the last patch was applied would never be
logged, because the system would immediately indicate that no upgrade was
pending.

* Move migration scripted tasks to the stream folder naming them .task.php.
* Add a MigrationTask abstract class to server as the base of migration
  tasks.
* Move the migration tasks from class.upgrader to the new task files

Allow the installation to use the same streams config and schema signatures
as the migrater

Allow the install to install mulitple database streams

Allow `streams.cfg` to be implied as 'core', which will default to only
upgrading the core osTicket stream.

Use signature to describe snapshots rather than md5

This patch allows the upgrader to upgrade multiple database update
'streams'. The main stream is renamed to 'core' to reflect changes made
to the config class. This will significantly ease customizations
requiring database customizations, and will make plugins requiring
database changes possible.

- timezone_offset was dropped from the config table at 1.7-dpr1
- upgrader: only apply five patches in one request
- upgrader: fix readPatchInfo to work correctly
- session: support migrating from 1.6 (again)
- config: support migrating from 1.6 (fallback)
- config: no default for 'isonline' setting
- config: fix SQL whitespace issue for schema signature fallbacks
- config: hash 1.6 versions in the schema signature lookup
- upgrader: fix logging bug in attachment migration

Remove clobber between queue() method and queue instance property (which is
the queue list). Also properly limit the number of attachments queued for
migration as indicated by the initial call to the queueAttachments() method.
Lastly, fix an issue where the first $limit items of the database cannot be
found on or read from the disk. Recurse through the queueAttachments()
method until the entire database is scanned or $limit items are placed in
the queue.

Implement attachment migration for attachments
Add support for more information stored about the patches in the individual
patche files with a Javadoc style syntax