Exchange lastCheque

[Eknir]

Based on a TODO in the code:
When two peers connect and they already have cheques submitted to each other before, they should exchange the last known cheque with each other.

Up for discussion: why would an issuer of a cheque do this? It is in his incentive to not exchange the last known cheque in the hope that a node loses his cheque.
Besides, if a peer peer has lost a cheque for whatever reason, the peer will be on-track again (in the current implementation) right after he receives a new cheque from the issuer (as the cheque amount is always cumulative).

My personal opinion is that sending last-know cheques at handshake seems like a pre-mature optimization and that we can wait for the network to be live to gather statistics on lost cheques and how nodes behave if the do.

[mortelli]

@ralph-pichler do you agree on deprecating this through #1887?

[ralph-pichler]

I don't think the PR addressed the things described here. But I don't think there is any plan for this exchange at this point, so maybe we should close this?