Separate auxiliary HTLC data from holder commitment transaction

We shouldn't track our `HTLCSource`s within our
`HolderCommitmentTransaction`s duplicatively for each `FundingScope`.
With splicing, we may have alternative holder commitment transactions,
but they must all have the same set of non-dust and dust HTLCs as the
pre-spliced commitment transaction. Different sets of HTLCs are only
possible with a change to the dust limit on commitment transactions,
which the splicing protocol does not currently support.

This commit moves the `nondust_htlc_sources` and `dust_htlcs` fields
out from each `FundingScope` into the `ChannelMonitor`, such that they
can be reused for each `FundingScope`. This remains as a backwards
compatible change, the underlying stored data is not changed, but where
it lives in memory is.

Author: wpaulino