1. Rename channel_funding_locked to channel_funding_depth in
channeld/channel_wire.csv.
2. Add minimum_depth in struct channel in common/initial_channel.h and
change corresponding init function: new_initial_channel().
3. Add confirmation_needed in struct peer in channeld/channeld.c.
4. Rename channel_tell_funding_locked to channel_tell_depth.
5. Call channel_tell_depth even if depth < minimum, and still call
lockin_complete in channel_tell_depth, iff depth > minimum_depth.
6. channeld ignore the channel_funding_depth unless its >
minimum_depth(except to update billboard, and set
peer->confirmation_needed = minimum_depth - depth).
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