Re-send FundingLocked in ChannelLink upon re-connect if at state zero

[Roasbeef]

Issue

The current specification draft describes the process of re-synchronizing channel state upon reconnection. As transports can be unreliable, we don't have a guarantee that the remote party actually received the last update we sent before the connection was closed. As a result, upon reconnection we'll attempt to retransmit the state necessary to reconcile any possible deviations in our state.

PR #231 implements the bulk of the retransmission logic but leaves one component un-implemented:

If next_local_commitment_number is 1 in both the channel_reestablish it sent and received, then the node MUST retransmit funding_locked, otherwise it MUST NOT. On reconnection, a node MUST ignore a redundant funding_locked if it receives one.

Steps to Completion

• Upon reconnection (as an intermediate step before Add retransmission subsystem in channel link #231 is merge) if the current state number o is zero within the ChannelLink's main goroutine then the FundingLocked message should be reconstructed, and re-sent to the remote peer.

  The FundingLocked message can be reconstructed by obtaining the the next commitmentPoint from the core channel state machine.

• Within the fundingManager when handling the FundingLocked message if this is a retransmitted message, then it should be ignored, rather than continuing normal processing. We can determine if this is a retransmission by examining the RemoteNextRevocation field for nil-ness

• Currently, within the fundingManagerwhen we try to send the FundingLocked message to the remote party, but they're not currently online, we simply exit early and fail to properly announce the channel. Instead, we should send a request to the server to be notified of when the target peer comes online again. In the case the peer re-connects, then we should wait up and resume duties as normal. This is also related to Implement block-based grace period for channel announcements  #305 as we'll need to decouple channel graph insertion from channel announcement as well.

[halseth]

Taking a look at this one :)

[halseth]

Can be closed re #328 :)

[Roasbeef]

Closing.