Store justifications obtained while warp syncing

[bkchr]

When a node warp syncs to the tip of the chain, it will need to download the justifications of every era change to proof the authority set changes. Currently we don't store these justifications and download them later. However, it would be smarter to directly store them as the node would then be instantly able to answer warp proof requests. We will also need to store the header of the blocks associated to these justifications.