Create settlement engine account after mutual peering

[kincaidoneil]

Unfortunately, #591 still has an issue: when peering two nodes, if the account that's prefunding is created before the other account, the settlement will be triggered immediately and then fail because the peer's account has not yet been created. Having the settlement engine backoff and retry isn't a great strategy, since the settlement engine has now idea why it failed or if/when it's messages will get through.

A better solution would be to only create the account on the settlement engine after the node knows the account has been created on the peer, which could be done via an interactive account creation process.