Implement savepoints migration

[grzesiek2010]

Problem description

In #5951 we have reworked the way we deal with savepoints. The new approach is based on a database and requires some kind of data migration to provide backward compatibility.

[seadowg]

My take is that we can't release without being able to migrate existing save points or else we risk losing people's data.

@grzesiek2010 is there anything we need to capture here other than the fact that a save point created before upgrading is still available after upgrading?