Update for channel phase changes

Moves the following methods (which, once unwound, don't actually depend on
anything but the ChannelContext) into ChannelContext:

- Channel::build_commitment_no_state_update
- Channel::send_commitment_no_state_update
- Channel::get_last_commitment_update_for_send
- Channel::signer_maybe_unblocked

Modifies `signer_unblocked` to use the phase rather than the channel.