Move to `Funded` after `funding_signed` rather than on funding

Move to `Funded` after `funding_signed` rather than on funding

Previously, channels were stored in different maps in `PeerState`
based on whether the funding had been set, keeping the keys across
the maps consistent (pre-funding temporary_channel_ids vs
funding-outpoint-based channel_ids). However, channels are now
stored in a single `channel_by_id` map, making that point moot.

Instead, here, we convert the `ChannelPhase` state transition
boundary to "once we have a `ChannelMonitor`", which makes more
sense now, and was actually the original proposed boundary.

This also requires calling `signer_maybe_unblocked` on a pre-funded
outbound channel, but that nicely also lets us limit the scope of
`FundingCreated` message generation, which we do in the next
commit.