Sort out `DiscardFunding` event with tests for V2 channels

[dunxen]

Doesn't have to be in this PR, but we definitely need to get our `DiscardFunding` event story together (with tests) before this ships.

Originally posted by @TheBlueMatt in #3137 (comment)

[TheBlueMatt]

We're gonna walk back our v2 enablement for 0.1, so this needs to happen for 0.2.