Don't attempt to resume channels if we already exchanged funding

Don't attempt to resume channels if we already exchanged funding

ddf75afd16 introduced the ability to re-exchange our `ChannelOpen`
after a peer disconnects if we didn't complete funding on our end.
It did not implement nor consider what would happen if we
re-connected after we created our own funding transactions, and
currently it panics (and even if it did not it would replay the
`FundingTransactionGenerated` event to users).

This addresses this in a temporary fashion to fix the immediate
panic for 0.0.123, allowing us to fix the issue more fully later.