cmd/swarm: add config file (#15548)

This commit adds a TOML configuration option to swarm. It reuses
the TOML configuration structure used in geth with swarm
customized items.

The commit:

* Adds a "dumpconfig" command to the swarm executable which
  allows printing the (default) configuration to stdout, which
  then can be redirected to a file in order to customize it.
* Adds a "--config <file>" option to the swarm executable which will
  allow to load a configuration file in TOML format from the
  specified location in order to initialize the Swarm node The
  override priorities are like follows: environment variables
  override command line arguments override config file override
  default config.

swarm/swarm.go

@@ -220,7 +220,7 @@ func (self *Swarm) Start(srv *p2p.Server) error {
 // stops all component services.
 func (self *Swarm) Stop() error {
 	self.dpa.Stop()
-	self.hive.Stop()
+	err := self.hive.Stop()
 	if ch := self.config.Swap.Chequebook(); ch != nil {
 		ch.Stop()
 		ch.Save()
@@ -230,7 +230,7 @@ func (self *Swarm) Stop() error {
 		self.lstore.DbStore.Close()
 	}
 	self.sfs.Stop()
-	return self.config.Save()
+	return err
 }
 
 // implements the node.Service interface
@@ -301,7 +301,6 @@ func (self *Swarm) SetChequebook(ctx context.Context) error {
 		return err
 	}
 	log.Info(fmt.Sprintf("new chequebook set (%v): saving config file, resetting all connections in the hive", self.config.Swap.Contract.Hex()))
-	self.config.Save()
 	self.hive.DropAll()
 	return nil
 }
@@ -314,10 +313,9 @@ func NewLocalSwarm(datadir, port string) (self *Swarm, err error) {
 		return
 	}
 
-	config, err := api.NewConfig(datadir, common.Address{}, prvKey, network.NetworkId)
-	if err != nil {
-		return
-	}
+	config := api.NewDefaultConfig()
+	config.Path = datadir
+	config.Init(prvKey)
 	config.Port = port
 
 	dpa, err := storage.NewLocalDPA(datadir)