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.

vendor/github.com/lib/pq/user_posix.go

@@ -0,0 +1,24 @@
+// Package pq is a pure Go Postgres driver for the database/sql package.
+
+// +build darwin dragonfly freebsd linux nacl netbsd openbsd solaris rumprun
+
+package pq
+
+import (
+	"os"
+	"os/user"
+)
+
+func userCurrent() (string, error) {
+	u, err := user.Current()
+	if err == nil {
+		return u.Username, nil
+	}
+
+	name := os.Getenv("USER")
+	if name != "" {
+		return name, nil
+	}
+
+	return "", ErrCouldNotDetectUsername
+}