Warp sync starting point should be a session change block

[tomaka]

When sending a warp sync request, if the target node has itself synced through a warp sync, it might not have finished downloading all blocks.

The only guarantee that we have is that it has finished downloading the header and justifications of blocks at session changes, since it had to download these for its warp sync.

Consequently, the block in a warp sync request must be a session change block.
Consequently, the checkpoint that we put in chain specs must be a session change block. Or, alternatively, the chain spec can contain both a session change block and a more recent block.