Do not panic if a peer learns our funding info before we fund

Do not panic if a peer learns our funding info before we fund

We'd previously assumed that LDK would receive
`funding_transaction_generated` prior to our peer learning the txid
and panicked if the peer tried to open a redundant channel to us
with the same funding outpoint.

While this assumption is generally safe, some users may have
out-of-band protocols where they notify their LSP about a funding
outpoint first, or this may be violated in the future with
collaborative transaction construction protocols, i.e. the upcoming
dual-funding protocol.