chanacceptor: Adding Chained and RPC acceptors

This commit introduces the chanacceptor package which is used
to determine, by a set of heuristics, which open channel messages
to accept and reject. Currently, two acceptors are implemented
via the ChannelAcceptor interface: ChainedAcceptor and RPCAcceptor.
The RPCAcceptor allows the RPC client to respond to the open channel
request, and the ChainedAcceptor allows a conjunction of acceptors
to be used.

chanacceptor/interface.go

@@ -0,0 +1,25 @@
+package chanacceptor
+
+import (
+	"github.com/btcsuite/btcd/btcec"
+	"github.com/lightningnetwork/lnd/lnwire"
+)
+
+// ChannelAcceptRequest is a struct containing the requesting node's public key
+// along with the lnwire.OpenChannel message that they sent when requesting an
+// inbound channel. This information is provided to each acceptor so that they
+// can each leverage their own decision-making with this information.
+type ChannelAcceptRequest struct {
+	// Node is the public key of the node requesting to open a channel.
+	Node *btcec.PublicKey
+
+	// OpenChanMsg is the actual OpenChannel protocol message that the peer
+	// sent to us.
+	OpenChanMsg *lnwire.OpenChannel
+}
+
+// ChannelAcceptor is an interface that represents a predicate on the data
+// contained in ChannelAcceptRequest.
+type ChannelAcceptor interface {
+	Accept(req *ChannelAcceptRequest) bool
+}