Fix PostgreSQL start when postmaster.pid is not removed after non-clean shutdown

When PostgreSQL is not terminated nicely, a pidfile is left behind in db_data,
which will prevent pg_ctl from starting a new PostgreSQL server.

We check for postmaster.pid file and verify that postgres server with PID
specified in the pidfile is not running anymore, then delete the pidfile to
allow pg_ctl to start a new server. If the postgres server is still running
(possibly after Akonadi server crash), we try to connect to it right away.

BUG: 286826
FIXED-IN: 1.12.2