PostgreSQL supports synchronous multi-master, MongoDB supports write concern, but this causes a performance penalty).
Anyways I doubt that "PostgreSQL supports synchronous multi-master"
Postgres core taken as such does not support multi-master. The fork of PostgreSQL called Postgres-XC somewhat does, for OLTP applications.
I know about PostgresXC, but I thought it is distributed database (similar to shards of mongoDB). [Though if I am correct there could be tables which are shared across different nodes, but that is not the best way of utilizing features of PostgresXC] I think it is not apt to call it "synchronous" (since there is no replication happening).