Within the network, it's important that when an HTLC forwarding failure
occurs, the recipient is notified in a timely manner in order to ensure
that errors are graceful and not unknown. For that reason with
accordance to BOLT №4 onion failure obfuscation have been added.
In this commit usage of the pending packet queue have been added.
This queue will consume the downstream packets if state machine return
the error that we do not have enough capacity for htlc in commitment
transaction. Upon receiving settle/fail payment descriptors - add htlc
have been removed, we release the slot, and process pending add htlc
requests.
Step #5 in making htlcManager (aka channelLink) testable:
Combine all that have been done so far and add test framework for channel
links which allow unit test:
* message ordering
* detect redundant messages
* single hop payment
* multihop payment
* several cancel payment scenarios
