Cleanup Db.php

What does this MR do?

Move any non-legacy code out of Db.php

• move session-related methods to Session.php
• refactor initSessionData to use gateways
• add new method db->insertOrUpdate($table, $data) that can be used to migrate REPLACE INTO queries

How confident are you it won't break things if deployed?

Seems good so far, but refactoring initSessionData my cause subtle bugs as the data is kept in cache.

Maybe it makes sense to reset all existing sessions on deploy, to make sure we get immediate feedback.

Links to related issues

#9 (closed)

Checklist

• added a test, or explain why one is not needed/possible...
• no unrelated changes
• asked someone for a code review
• joined #foodsharing-beta channel at https://slackin.yunity.org
• added an entry to CHANGELOG.md (description, merge request link, username(s))