ICS003 Connection Upgrades: In-flight packets

[AdityaSripal]

Ref: #725

In-flight packet handling for connection upgrades is more tricky than channel upgrades. Just as before assuming packet verification does not break, then once the connection upgrade completes; all packet verification resumes (either timeouts or receives).

However, if the connection upgrade happens to break packet verification (e.g. the proof paths in ICS24 get changed), then in-flight packets will fail verification but it will not be recoverable in-protocol. It will require manual migration or recovery to deal with this issue.

Luckily, connection upgrades are intended for core developers to be able to change the core IBC stack. So it is incumbent on future core teams to upgrade the stack in ways that are either backwards compatible or provide the requisite migration code to end chains that may want to perform the upgrade.

This must be clearly documented in the spec, especially since the potential downside of a poorly planned connection upgrade is worse than channel upgrade.