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/rpcacceptor.go

@@ -0,0 +1,27 @@
+package chanacceptor
+
+// RPCAcceptor represents the RPC-controlled variant of the ChannelAcceptor.
+// One RPCAcceptor allows one RPC client.
+type RPCAcceptor struct {
+	acceptClosure func(req *ChannelAcceptRequest) bool
+}
+
+// Accept is a predicate on the ChannelAcceptRequest which is sent to the RPC
+// client who will respond with the ultimate decision. This assumes an accept
+// closure has been specified during creation.
+//
+// NOTE: Part of the ChannelAcceptor interface.
+func (r *RPCAcceptor) Accept(req *ChannelAcceptRequest) bool {
+	return r.acceptClosure(req)
+}
+
+// NewRPCAcceptor creates and returns an instance of the RPCAcceptor.
+func NewRPCAcceptor(closure func(*ChannelAcceptRequest) bool) *RPCAcceptor {
+	return &RPCAcceptor{
+		acceptClosure: closure,
+	}
+}
+
+// A compile-time constraint to ensure RPCAcceptor implements the ChannelAcceptor
+// interface.
+var _ ChannelAcceptor = (*RPCAcceptor)(nil)