Add HTLC/extra data in LocalCommitmentTransaction from construction

1107ab06c33bd360bdee7ee64f4b690e753003f6 introduced some additional
metadata, including per-HTLC data in LocalCommitmentTransaction. To
keep diff reasonable it did so in ChannelMonitor after the
LocalCommitmentTransaction had been constructed and passed over the
wall, but there's little reason to do so - we should just be
constructing them with the data from the start, filled in by Channel.

This cleans up some internal interfaces a bit, slightly reduces
some data duplication and moves us one step forward to exposing
the guts of LocalCommitmentTransaction publicly in a sensible way.