This is to support writing raw ints directly to db keys / db values as a
space saving measure. QDataStream, while platform neutral, adds its own
marker/size bytes to the serialization which we do not need always.
Our DB format is now endian and architecture-specific, but that's ok.
Users can resynch if they switch machine architectures.
We now save the "chain" variable to the meta table. We also panic if
the headers we are reading from bitcoind are behind what we have (todo:
handle this elegantly if there actually was a reorg?)
