Replace index with constraint

#700

db/migrate/20180212152810_add_epp_sessions_session_id_unique_constraint.rb

@@ -0,0 +1,13 @@
+class AddEppSessionsSessionIdUniqueConstraint < ActiveRecord::Migration
+  def up
+    execute <<-SQL
+      ALTER TABLE epp_sessions ADD CONSTRAINT unique_session_id UNIQUE (session_id)
+    SQL
+  end
+
+  def down
+    execute <<-SQL
+      ALTER TABLE epp_sessions DROP CONSTRAINT unique_session_id
+    SQL
+  end
+end

db/migrate/20180212154731_remove_epp_sessions_session_id_unique_index.rb

@@ -0,0 +1,5 @@
+class RemoveEppSessionsSessionIdUniqueIndex < ActiveRecord::Migration
+  def change
+    remove_index :epp_sessions, name: :index_epp_sessions_on_session_id
+  end
+end