gossip: delay considering a channel edge deleted for 12-blocks (#1004)

Allows us time to propagate a splice update (new channel_announcement)

Suggested-By: @t-bast
Co-Authored: @ddustin

07-routing-gossip.md

@@ -224,7 +224,7 @@ The receiving node:
  - otherwise:
  - SHOULD store this `channel_announcement`.
  - once its funding output has been spent OR reorganized out:
- - SHOULD forget a channel.
+ - SHOULD forget a channel after a 12-block delay.
 
 ### Rationale
 
@@ -249,6 +249,10 @@ optional) features will have _odd_ feature bits, while incompatible features
 will have _even_ feature bits
 (["It's OK to be odd!"](00-introduction.md#glossary-and-terminology-guide)).
 
+A delay of 12-blocks is used when forgetting a channel on funding output spend
+as to permit a new `channel_announcement` to propagate which indicates this
+channel was spliced.
+
 ## The `node_announcement` Message
 
 This gossip message allows a node to indicate extra data associated with it, in