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/interface.go

@@ -62,6 +62,10 @@ type ChannelAcceptResponse struct {
 	// MinAcceptDepth is the minimum depth that the initiator of the
 	// channel should wait before considering the channel open.
 	MinAcceptDepth uint16
+
+	// ZeroConf indicates that the fundee wishes to send min_depth = 0 and
+	// request a zero-conf channel with the counter-party.
+	ZeroConf bool
 }
 
 // NewChannelAcceptResponse is a constructor for a channel accept response,
@@ -72,7 +76,7 @@ type ChannelAcceptResponse struct {
 func NewChannelAcceptResponse(accept bool, acceptErr error,
 	upfrontShutdown lnwire.DeliveryAddress, csvDelay, htlcLimit,
 	minDepth uint16, reserve btcutil.Amount, inFlight,
-	minHtlcIn lnwire.MilliSatoshi) *ChannelAcceptResponse {
+	minHtlcIn lnwire.MilliSatoshi, zeroConf bool) *ChannelAcceptResponse {
 
 	resp := &ChannelAcceptResponse{
 		UpfrontShutdown: upfrontShutdown,
@@ -82,6 +86,7 @@ func NewChannelAcceptResponse(accept bool, acceptErr error,
 		HtlcLimit:       htlcLimit,
 		MinHtlcIn:       minHtlcIn,
 		MinAcceptDepth:  minDepth,
+		ZeroConf:        zeroConf,
 	}
 
 	// If we want to accept the channel, we return a response with a nil