PostgreSQL is crashing on GPRD

I noticed PostgreSQL has been crashing on GPRD. I finally saw why:

2018-03-06_14:14:42.80297 postgres-01-db-gprd postgresql: FATAL:  semctl(47644828, 3, SETVAL, 0) failed: Invalid argument
2018-03-06_14:14:42.80358 postgres-01-db-gprd postgresql: LOG:  server process (PID 7357) exited with exit code 1

This relates to https://gitlab.com/gitlab-com/infrastructure/issues/1320.

We really should make the gitlab-psql user a system user (UID 100-500) instead of UID 1101.