Big wiring re-org for funding-continue
In openingd, we move the 'persistent' state (their basepoints,
pubkey, and the minimum_depth requirement for the opening tx) into
the state object. We also look to keep code-reuse between
'continue' and normal 'fundchannel' as high as possible. Both
of these call the same 'fundchannel_reply' at the end.
In opening_control.c, we remap fundchannel_reply such that it is
now aware of the difference between an external/internally funded
channel open. It's the same return path, with the difference that
one finishes making and broadcasting the funding transaction; the
other is skips this.
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