Add auto update of checkpoints to the openethereum spec

[zemse]

Checkpoints from the plasma contract can be fetched by kami and placed in the specs file before running the openethereum node.

openethereum/openethereum@7c7a995#diff-d535917a05d96f2ba39c32c9e1632893

[zemse]

This can be postponed for now, but the block hashes to be added in the proposal generation of kami.

[zemse]

Added lastBlockHash to the bunch (665edb3)