Refactor room server

Serialize and save room state in database
Get room state from database when creating a room server

priv/repo/migrations/20210729143750_change_room_state_to_array.exs

@@ -0,0 +1,10 @@
+defmodule MatrixServer.Repo.Migrations.ChangeRoomStateToArray do
+  use Ecto.Migration
+
+  def change do
+    alter table(:rooms) do
+      remove :state, :map, default: %{}, null: false
+      add :state, {:array, {:array, :string}}, default: [], null: false
+    end
+  end
+end