docs+lnrpc: rename rpc.proto to lightning.proto

To avoid a naming conflict with etcd, we rename our very generic
rpc.proto to lightning.proto to match the service name that's declared
within. This will break many external tutorials and possibly also our
API docs but the change needs to be done eventually.

lnrpc/gen_protos.sh

@@ -6,7 +6,7 @@ set -e
 function generate() {
   echo "Generating root gRPC server protos"
   
-  PROTOS="rpc.proto walletunlocker.proto stateservice.proto **/*.proto"
+  PROTOS="lightning.proto walletunlocker.proto stateservice.proto **/*.proto"
   
   # For each of the sub-servers, we then generate their protos, but a restricted
   # set as they don't yet require REST proxies, or swagger docs.