Convert the database interface from the stdlib `database/sql` package to
the GORM object relational mapper.
GORM is also used by the Manager, and thus with this change both Worker
and Manager have a uniform way of accessing their databases.
The upstream buffer takes care of two things: communication with Flamenco
Manager (first context) and buffering things in a SQLite database (second
context). This commit separates those two contexts, so that shutting down
the application isn't going to prevent buffering things in the database.
The add-on code was copy-pasted from other addons and used the GPL v2
license, whereas by accident the LICENSE text file had the GNU "Affero" GPL
license v3 (instead of regular GPL v3).
This is now all streamlined, and all code is licensed as "GPL v3 or later".
Furthermore, the code comments just show a SPDX License Identifier
instead of an entire license block.
Instead of returning an error "error doing X", just return "doing X". The
fact that it's returned as an error object says enough about that it's
an error.
This also makes it easier to chain error messages, without seeing the
word "error" in every part of the chain.
