Support multiple update streams

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.
Get the upgrader to actually work
Allow streams to be registered in streams.cfg
Add start of documentation for database migration

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