Trying to do the upgrade of the hdd cluster using the pg_upgrade method.

1. initdb the new cluster : sudo LC_ALL=C.UTF-8 pg_createcluster -d /srv/softwareheritage/postgres-hdd/9.6/hdd 9.6 hdd
2. merge config from old cluster to new (postgresql.conf, pg_hba.conf)
3. create symlink to config in the new cluster for pg_upgrade : ln -s /etc/postgresql/9.6/hdd/postgresql.conf /srv/softwareheritage/postgres-hdd/9.6/hdd/postgresql.conf
4. check that pg_upgrade can perform the upgrade. this makes sure that both clusters are compatible : /usr/lib/postgresql/9.6/bin/pg_upgrade --check --link --old-bindir /usr/lib/postgresql/9.5/bin --new-bindir /usr/lib/postgresql/9.6/bin --old-datadir /srv/softwareheritage/postgres-hdd/9.5/hdd/ --new-datadir /srv/softwareheritage/postgres-hdd/9.6/hdd --old-port 5435 --new-port 5437
5. perform the upgrade (remove --check from previous commandline) and wait