Replace use of HolderSignedTx with HolderCommitment

As we move to a custom commitment transactions world, we'll no longer be
able to rely on `HolderSignedTx` and should instead track the actual
commitment transaction itself. This commit does so by introducing a new
`HolderCommitment` struct, which we'll use as a replacement to
`HolderSignedTx` to hold all relevant holder commitment data, including
the `HolderCommitmentTransaction`, without data duplication.

Luckily, this is a backwards and forwards compatible change due to the
`OnchainTxHandler` tracking the latest `HolderCommitmentTransaction`s.
In the future, we aim to remove them from the `OnchainTxHandler`
entirely and begin storing them at the `ChannelMonitor` level.

Aside from how the data is represented internally, there shouldn't be
any functional changes within this patch.

Author: wpaulino