The NrBuilds table tracks the value of ‘select count(*) from Builds
where finished = 0’, keeping it up to date via a trigger. This is
necessary to make the /all page fast, since otherwise it needs to do a
sequential scan on the Builds table.
This gets rid of the openHydraDB function and ensures that we
open the database in a consistent way.
Also drop the PostgreSQL sequence hacks. They don't seem to be
necessary anymore.
For schema upgrades, hydra-init executes the files
src/sql/upgrade-<N>.sql, each of which upgrades the schema from
version N-1 to N. The upgrades are wrapped in a transaction.
