Cross-msgs should be batched if checkpoint signing window passes?

[adlrocha]

In the current implementation, the SCA batches in the checkpoint template all the cross-msgs that need to be propagated in the checkpoint. After the checkpointing window passes, the signing window opens for an additional period in which miners can populate the template, sign it, and submit it to the parent. When the signing window passes, the checkpoint template is overwritten with the next one, which means that if the checkpoint is not successfully signed the cross-messages for that checkpoint are lost.

We should buffer the checkpoint template and check in the parent if the checkpoint has been successfully propagated so we don't lose cross-message for propagation (this may be partially fixed in #144)