(#3182) - WIP - avoid on-the-fly idb migrations

I was getting tired of the code complexity with
having to figure out whether `metadata.winningRev`
and `metadata.seq` were there or not. I prefer to
just migrate everything to be sure.

This also removes the docStore's 'seq' index, which
we weren't using and which apparently doesn't offer
any performance benefit.
^ We need a test to check that we're actually ignoring `opts.since` when `descending=true`, but I have another issue open for `descending=true` problems. In any case, I simplified the code here.