Reorganize code into server/{api,grpc,shared} packages (#337)

* move api code to server/api
* move grpc server for agent communication to server/grpc
* move server.Config to server/config.go as it is used by both server/api and server/grpc
* move shared code used by server/api and server/grpc to server/shared

cmd/server/main.go

@@ -31,7 +31,7 @@ func main() {
 	app.Name = "woodpecker-server"
 	app.Version = version.String()
 	app.Usage = "woodpecker server"
-	app.Action = server
+	app.Action = loop
 	app.Flags = flags
 	app.Before = before