Upgrade to PostgreSQL 10.0

Geo would benefit from having PostgreSQL 10 for its aggregate pushdown support: https://www.enterprisedb.com/blog/postgresql-aggregate-push-down-postgresfdw. Right now because PostgreSQL 9.6 supports join pushdowns, we are using an ugly INNER JOIN with millions of VALUES.

In 11.11, we will provide both PG 9.6 and 10.0 in the omnibus package with the ability to upgrade to PG 10.0 manually. Fresh GitLab installations will still have PG 9.6. GitLab upgrades to 11.11 will not touch existing PG version.

/cc: @yorickpeterse, @abrandl, @_stark