class ActiveRecord::ConnectionAdapters::ConnectionPool

calling +checkout+ on this pool.
+conn+: an AbstractAdapter object, which was obtained by earlier by

no longer need this connection.
Check-in a database connection back into the pool, indicating that you

def checkin(conn)
  synchronize do
    conn.run_callbacks :checkin do
      conn.expire
    end
    release conn
    @available.add conn
  end
end