If you want multi master in Postgres, I think BDR is going to be your best option, but the version for PG 10+ isn't open source so you'll have to pay for it. We're using the open source version on PG 9.4 currently in production, it's worked fine so far.

If you're just looking for a hot standby and dont need a multi master setup, you can set those up just with pg. https://www.postgresql.org/docs/9.4/hot-standby.html