Now the Worker and the Manager share the same database initialisation
code (enabling foreign key constraints + write-ahead logging).
The foreign key constraints were already enabled before, but now it's done
with (a copy of) the same code as the Manager.
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.
