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.

funding/interfaces.go

@@ -19,3 +19,31 @@ type Controller interface {
 	// represents a pending channel in the Controller implementation.
 	IsPendingChannel([32]byte, lnpeer.Peer) bool
 }
+
+// aliasHandler is an interface that abstracts the managing of aliases.
+type aliasHandler interface {
+	// RequestAlias lets the funding manager request a unique SCID alias to
+	// use in the funding_locked message.
+	RequestAlias() (lnwire.ShortChannelID, error)
+
+	// PutPeerAlias lets the funding manager store the received alias SCID
+	// in the funding_locked message.
+	PutPeerAlias(lnwire.ChannelID, lnwire.ShortChannelID) error
+
+	// GetPeerAlias lets the funding manager lookup the received alias SCID
+	// from the funding_locked message. This is not the same as GetAliases
+	// which retrieves OUR aliases for a given channel.
+	GetPeerAlias(lnwire.ChannelID) (lnwire.ShortChannelID, error)
+
+	// AddLocalAlias persists an alias to an underlying alias store.
+	AddLocalAlias(lnwire.ShortChannelID, lnwire.ShortChannelID, bool) error
+
+	// GetAliases returns the set of aliases given the main SCID of a
+	// channel. This SCID will be an alias for zero-conf channels and will
+	// be the confirmed SCID otherwise.
+	GetAliases(lnwire.ShortChannelID) []lnwire.ShortChannelID
+
+	// DeleteSixConfs removes the passed SCID from one of the underlying
+	// alias store's indices.
+	DeleteSixConfs(lnwire.ShortChannelID) error
+}