Add #521 - second attempt

Sure! See (the somewhat badly named) `TestNodeConfig`. It loads configs of versions 1 though 4 and verifies that they all result in the same config struct, i.e. the migrations do what is expected. I'd suggest adding the rescan interval as an option to each of those versions, in the manner that is expected for 1-3 and the new one in 4. In each case, the `expectedRepos` should contain the rescan interval.