channeldb: relax bucket assumptions for latest db migration
channeldb: relax bucket assumptions for latest db migration

In this commit, we no longer assume that the bucket hierarchy has been
created properly when applying the latest DB migration. On older nodes
that never obtained a channel graph, or updated _before_ the query sync
stuff was added, then they're missing buckets that the migration expects
them to have.

We fix this by simply creating the buckets as we go, if needed.

aakselrod/lnd,LightningNetwork/lnd,aakselrod/lnd,LightningNetwork/lnd