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/syncer.go

@@ -131,9 +131,8 @@ type SyncParams struct {
 }
 
 // constructor with default values
-func NewSyncParams(bzzdir string) *SyncParams {
+func NewDefaultSyncParams() *SyncParams {
 	return &SyncParams{
-		RequestDbPath:      filepath.Join(bzzdir, "requests"),
 		RequestDbBatchSize: requestDbBatchSize,
 		KeyBufferSize:      keyBufferSize,
 		SyncBufferSize:     syncBufferSize,
@@ -144,6 +143,12 @@ func NewSyncParams(bzzdir string) *SyncParams {
 	}
 }
 
+//this can only finally be set after all config options (file, cmd line, env vars)
+//have been evaluated
+func (self *SyncParams) Init(path string) {
+	self.RequestDbPath = filepath.Join(path, "requests")
+}
+
 // syncer is the agent that manages content distribution/storage replication/chunk storeRequest forwarding
 type syncer struct {
 	*SyncParams                     // sync parameters