lnrpc: add the devrpc development only subserver + devrpc.ImportGraph

This commits adds the devrpc package which implements a subserver that
adds clean separation for RPC calls useful for development and
debugging. This subserver is only compiled in if the dev tag is set.
Furthermore the commit adds the devrpc.ImportGraph call which can
import a graph dump obtained from another node by calling DescribeGraph.
Since the graph dump does not include the auth proofs, the imported
channels will be considered private.

lnrpc/devrpc/dev.proto

@@ -0,0 +1,18 @@
+syntax = "proto3";
+
+import "lightning.proto";
+
+package devrpc;
+
+option go_package = "github.com/lightningnetwork/lnd/lnrpc/devrpc";
+
+service Dev {
+    /* lncli: `importgraph`
+    ImportGraph imports a ChannelGraph into the graph database. Should only be
+    used for development.
+    */
+    rpc ImportGraph (lnrpc.ChannelGraph) returns (ImportGraphResponse);
+}
+
+message ImportGraphResponse {
+}