Use `KeysInterface::read_chan_signer` for all channel keys deser

Use `KeysInterface::read_chan_signer` for all channel keys deser

This drops any direct calls to a generic `ChannelKeys::read()` and
replaces it with the new `KeysInterface::read_chan_signer()`. Still,
under the hood all of our own `KeysInterface::read_chan_signer()`
implementations simply call out to a `Readable::read()` implemention.