Ensure height is set on block as well as the node.

Even though the code currently makes heavy use of nodes which will have
the appropriate height, blocks which did not come from the database will
not have a height set.  As a result, this could possibly result in a
height of 0 being used when unintended.  By setting the block height in
the block as soon as its known (regardless of the source), we can future
proof against bugs that would likely be hard to track down.