server+funding: allow scid-alias, zero-conf chantypes, scid-alias

feature-bit channels

This allows opening zero-conf chan-type, scid-alias chan-type, and
scid-alias feature-bit channels. scid-alias chan-type channels are
required to be private. Two paths are available for opening a zero-conf
channel:

* explicit chan-type negotiation
* LDK carve-out where chan-types are not used, LND is on the
  receiving end, and a ChannelAcceptor is used to enable zero-conf

When a zero-conf channel is negotiated, the funding manager:
* sends a FundingLocked with an alias
* waits for a FundingLocked from the remote peer
* calls addToRouterGraph to persist the channel using our alias in
  the graph. The peer's alias is used to send them a ChannelUpdate.
* wait for six confirmations. If public, the alias edge in the
  graph is deleted and replaced (not atomically) with the confirmed
  edge. Our policy is also read-and-replaced, but the counterparty's
  policy won't exist until they send it to us.

When a scid-alias-feature channel is negotiated, the funding manager:
* sends a FundingLocked with an alias:
* calls addToRouterGraph, sends ChannelUpdate with the confirmed SCID
  since it exists.
* when six confirmations occurs, the edge is deleted and re-inserted
  since the peer may have sent us an alias ChannelUpdate that we are
  storing in the graph.

Since it is possible for a user to toggle the scid-alias-feature-bit
to on while channels exist in the funding manager, care has been taken
to ensure that an alias is ALWAYS sent in the funding_locked message
if this happens.

chanacceptor/rpcacceptor.go

@@ -107,7 +107,7 @@ func (r *RPCAcceptor) Accept(req *ChannelAcceptRequest) *ChannelAcceptResponse {
 	// Create a rejection response which we can use for the cases where we
 	// reject the channel.
 	rejectChannel := NewChannelAcceptResponse(
-		false, errChannelRejected, nil, 0, 0, 0, 0, 0, 0,
+		false, errChannelRejected, nil, 0, 0, 0, 0, 0, 0, false,
 	)
 
 	// Send the request to the newRequests channel.
@@ -216,6 +216,7 @@ func (r *RPCAcceptor) receiveResponses(errChan chan error,
 			MaxHtlcCount:    resp.MaxHtlcCount,
 			MinHtlcIn:       resp.MinHtlcIn,
 			MinAcceptDepth:  resp.MinAcceptDepth,
+			ZeroConf:        resp.ZeroConf,
 		}
 
 		// We have received a decision for one of our channel
@@ -348,6 +349,7 @@ func (r *RPCAcceptor) sendAcceptRequests(errChan chan error,
 				btcutil.Amount(resp.ReserveSat),
 				lnwire.MilliSatoshi(resp.InFlightMaxMsat),
 				lnwire.MilliSatoshi(resp.MinHtlcIn),
+				resp.ZeroConf,
 			)
 
 			// Delete the channel from the acceptRequests map.