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_windows.go

@@ -0,0 +1,27 @@
+// Package pq is a pure Go Postgres driver for the database/sql package.
+package pq
+
+import (
+	"path/filepath"
+	"syscall"
+)
+
+// Perform Windows user name lookup identically to libpq.
+//
+// The PostgreSQL code makes use of the legacy Win32 function
+// GetUserName, and that function has not been imported into stock Go.
+// GetUserNameEx is available though, the difference being that a
+// wider range of names are available.  To get the output to be the
+// same as GetUserName, only the base (or last) component of the
+// result is returned.
+func userCurrent() (string, error) {
+	pw_name := make([]uint16, 128)
+	pwname_size := uint32(len(pw_name)) - 1
+	err := syscall.GetUserNameEx(syscall.NameSamCompatible, &pw_name[0], &pwname_size)
+	if err != nil {
+		return "", ErrCouldNotDetectUsername
+	}
+	s := syscall.UTF16ToString(pw_name)
+	u := filepath.Base(s)
+	return u, nil
+}