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/storage/netstore.go

@@ -57,15 +57,21 @@ type StoreParams struct {
 	Radius        int
 }
 
-func NewStoreParams(path string) (self *StoreParams) {
+//create params with default values
+func NewDefaultStoreParams() (self *StoreParams) {
 	return &StoreParams{
-		ChunkDbPath:   filepath.Join(path, "chunks"),
 		DbCapacity:    defaultDbCapacity,
 		CacheCapacity: defaultCacheCapacity,
 		Radius:        defaultRadius,
 	}
 }
 
+//this can only finally be set after all config options (file, cmd line, env vars)
+//have been evaluated
+func (self *StoreParams) Init(path string) {
+	self.ChunkDbPath = filepath.Join(path, "chunks")
+}
+
 // netstore contructor, takes path argument that is used to initialise dbStore,
 // the persistent (disk) storage component of LocalStore
 // the second argument is the hive, the connection/logistics manager for the node