OMG-421 - Allow OMG.DB database migrations

[boolafish]

Porting this from Pivotal Trackers as OMG-421 is referenced in the code.

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description from Pivotal Tracker 👇

So that we're not exposed errors related to running new versions of the software on old data, and don't have to resync (or ditch child chain data) to fix them

[InoMurko]

Childchain is being addressed.