0.4.0

• [BREAKING] Storex.mutate/3 is no longer based on session_id
• [BREAKING] Store.init/2 callback now need to return {:ok, state} | {:ok, state, key} | {:error, reason}
• [BREAKING] Remove custom Registry logic
• [BREAKING] Remove connection callback from javascript client
• New registry mechanism provides distributed mutations across the cluster
• Fix terminate callback in Storex.Handler.Plug
• Added three callbacks to frontend client onConnected, onError and onDisconnected