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/oid/doc.go

@@ -0,0 +1,6 @@
+// Package oid contains OID constants
+// as defined by the Postgres server.
+package oid
+
+// Oid is a Postgres Object ID.
+type Oid uint32