DRY `funding_created()` and `funding_signed()` for V1 channels

DRY `funding_created()` and `funding_signed()` for V1 channels

There is a decent amount of shared code in these two methods so we make
an attempt to share that code here by introducing the
`InitialRemoteCommitmentReceiver` trait. This trait will also come in
handy when we need similar commitment_signed handling behaviour for
dual-funded channels.