lnrpc: add GetState to stateservice (v1/state)

This commit adds a new GetState call to the state service in order to
make leader election integrate simply with k8s using a readinessProbe.

lnrpc/stateservice.proto

@@ -29,6 +29,10 @@ service State {
     // state will always be delivered immediately.
     rpc SubscribeState (SubscribeStateRequest)
         returns (stream SubscribeStateResponse);
+
+    // GetState returns the current wallet state without streaming further
+    // changes.
+    rpc GetState (GetStateRequest) returns (GetStateResponse);
 }
 
 enum WalletState {
@@ -46,3 +50,10 @@ message SubscribeStateRequest {
 message SubscribeStateResponse {
     WalletState state = 1;
 }
+
+message GetStateRequest {
+}
+
+message GetStateResponse {
+    WalletState State = 1;
+}