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.

lnrpc/lightning.proto

@@ -979,6 +979,13 @@ message ChannelAcceptResponse {
     The number of confirmations we require before we consider the channel open.
     */
     uint32 min_accept_depth = 10;
+
+    /*
+    Whether the responder wants this to be a zero-conf channel. This will fail
+    if either side does not have the scid-alias feature bit set. The minimum
+    depth field must be zero if this is true.
+    */
+    bool zero_conf = 11;
 }
 
 message ChannelPoint {