lightningd/opening_control.c: Remove 'Try fundchannel_cancel again' error.

[ZmnSCPxj]

Changelog-Changed: fundchannel_cancel will now succeed even when executed while a fundchannel_complete is ongoing; in that case, it will be considered as cancelling the funding after the fundchannel_complete succeeds.

Let me introduce the concept of "Sequential Consistency":
All operations on parallel processes form a single total order agreed upon by all processes.

So for example, suppose we have parallel invocations of fundchannel_complete and fundchannel_cancel:

                      +--[fundchannel_complete]-->
                      |
--[fundchannel_start]-+
                      |
                      +--[fundchannel_cancel]---->

What "Sequential Consistency" means is that the above parallel operations can be serialized as a single total order as:

--[fundchannel_start]--[fundchannel_complete]--[fundchannel_cancel]-->

Or:

--[fundchannel_start]--[fundchannel_cancel]--[fundchannel_complete]-->

In the first case, fundchannel_complete succeeds, and the fundchannel_cancel invocation also succeeds, sending an error to the peer to make them forget the chanel.

In the second case, fundchannel_cancel succeeds, and the succeeding fundchannel_complete invocation fails, since the funding is already cancelled and there is nothing to complete.

Note that in both cases, fundchannel_cancel always succeeds.

Unfortunately, prior to this commit, fundchannel_cancel could fail with a Try fundchannel_cancel again error if the fundchannel_complete is ongoing when the fundchannel_cancel is initiated.
This violates Sequential Consistency, as there is no single total order that would have caused fundchannel_cancel to fail.

This commit is a minimal patch which just reschedules fundchannel_cancel to occur after any fundchannel_complete that is ongoing.

[rustyrussell]

Ack 6f35452

Thanks! This is consistent with the modern cancel semantics. Any API which says "try again later" is always suspect!