postgresql systemd service fails to start only on boot but not manually

Hello everyone,

I'm having trouble with my PostgreSQL Systemd service and I wonder
whether this is a problem with the software or some problem with the
packaging or build.

My server fails to start PostgreSQL only on boot, if I restart it
manually afterwards it doesn't have any problem starting. Here is the
log extracted from the journal:

```
2018-09-21 20:46:40.028 CEST [306] LOG: listening on IPv4 address "127.0.0.1", port 5432
2018-09-21 20:46:40.036 CEST [306] LOG: listening on Unix socket "/run/postgresql/.s.PGSQL.5432"
2018-09-21 20:46:40.233 CEST [337] LOG: database system was shut down at 2018-09-21 20:46:21 CEST
2018-09-21 20:48:10.441 CEST [352] WARNING: worker took too long to start; canceled
2018-09-21 20:49:10.469 CEST [352] WARNING: worker took too long to start; canceled
2018-09-21 20:49:10.478 CEST [306] LOG: database system is ready to accept connections
2018-09-21 20:49:10.486 CEST [306] LOG: received fast shutdown request
2018-09-21 20:49:10.497 CEST [306] LOG: aborting any active transactions
2018-09-21 20:49:10.498 CEST [637] FATAL: terminating autovacuum process due to administrator command
2018-09-21 20:49:10.500 CEST [306] LOG: worker process: logical replication launcher (PID 636) exited with code 1
2018-09-21 20:49:10.501 CEST [349] LOG: shutting down
2018-09-21 20:49:10.518 CEST [306] LOG: database system is shut down
```

Any Ideas? I'd really like to fix this since now after every reboot I have to
remember to manually restart it.

Thanks.