Add PostgreSQL notifier (#2739) (#2824)

* The user is required to specify a table name and database connection
  information in the configuration file.

* INSERTs and DELETEs are done via prepared statements for speed.

* Assumes a table structure, and requires PostgreSQL 9.5 or above due to
  the use of UPSERT.

* Creates the table if it does not exist with the given table name using
  a query like:

    CREATE TABLE myminio (
        key varchar PRIMARY KEY,
        value JSONB
    );

* Vendors some required libraries.

cmd/globals.go

@@ -30,7 +30,7 @@ const (
 
 // minio configuration related constants.
 const (
-	globalMinioConfigVersion = "8"
+	globalMinioConfigVersion = "9"
 	globalMinioConfigDir     = ".minio"
 	globalMinioCertsDir      = "certs"
 	globalMinioCertFile      = "public.crt"