Restore timeouts from previous run

This is quite a big update, the new feature is that when the
program starts it loads the next update time from the db if
present. If so it resumes the timers from there. Maximum wait
time is currently capped at 24h.

TimerOroApi is not templated on methods anymore, rather on
the new DBOperation enum. This is so that it has that enum
value to pass it to other functions. This could've been a
separate commit, but wth...

Magic enum allows me to iterate over an enum's values at
build time, it's useful for building the query in OriginsDB
for the SELECT in the access table
(see make_select_last_access_str()).

src/timer_oro_api.cpp

@@ -0,0 +1,21 @@
+/* Copyright 2020, Michele Santullo
+ * This file is part of orotool.
+ *
+ * Orotool is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * Orotool is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with Orotool.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include "timer_oro_api.hpp"
+
+namespace duck {
+} //namespace duck