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/network/hive.go

@@ -70,19 +70,25 @@ type HiveParams struct {
 	*kademlia.KadParams
 }
 
-func NewHiveParams(path string) *HiveParams {
-	kad := kademlia.NewKadParams()
+//create default params
+func NewDefaultHiveParams() *HiveParams {
+	kad := kademlia.NewDefaultKadParams()
 	// kad.BucketSize = bucketSize
 	// kad.MaxProx = maxProx
 	// kad.ProxBinSize = proxBinSize
 
 	return &HiveParams{
 		CallInterval: callInterval,
-		KadDbPath:    filepath.Join(path, "bzz-peers.json"),
 		KadParams:    kad,
 	}
 }
 
+//this can only finally be set after all config options (file, cmd line, env vars)
+//have been evaluated
+func (self *HiveParams) Init(path string) {
+	self.KadDbPath = filepath.Join(path, "bzz-peers.json")
+}
+
 func NewHive(addr common.Hash, params *HiveParams, swapEnabled, syncEnabled bool) *Hive {
 	kad := kademlia.New(kademlia.Address(addr), params.KadParams)
 	return &Hive{