Build `per_peer_state` immediately in `ChannelManager` deser

Build `per_peer_state` immediately in `ChannelManager` deser

Instead of first building a map from peers to a list of channels
then pulling out of that to build the `per_peer_state`, we build
`per_peer_state` immediately and store channels in it immediately.

This avoids an unnecessary map indirection but also gives us
access to the new fields in `per_peer_state` when reading
`Channel`s which we'll need in a coming commit.