Enable connecting to postgres server via unix socket

Unix socket is the quickest possible connection to postgres server, so it's important to enable this possibility in program. The method init in hive/server/db.py doesn't handle connection string enabling connection via unix socket, like this for instance:

postgresql+psycopg2://my_user:my_password@/my_dbname?host=/var/run/postgresql&port=5432

In other places in code connecting via unix sockets is handled correctly.