[fix] Reset Bitcoin Observation Heights

Unless I've misunderstood something this will be required in order to unhalt Bitcoin. Since there was an error in the store migration (5f111012) that used the Thorchain height instead of the Bitcoin height, and Bifrost uses this to determine the start height of the scanner (blockscanner.go#L290) at init.

No great way to test the observe height reset on the nodes since it is not stored in genesis, but I'm manually testing in mocknet and will confirm in comment here.