cmd/swarm, swarm/api/client: add HTTP API client and 'swarm ls' command (#3742)

This adds a swarm ls command which lists files and directories stored in a
manifest. Rather than listing all files, it uses "directory prefixes" in case there are a
lot of files in a manifest but you just want to traverse it.

This also includes some refactoring to the tests and the introduction of a
swarm/api/client package to make things easier to test.

swarm/testutil/http.go

@@ -0,0 +1,56 @@
+package testutil
+
+import (
+	"io/ioutil"
+	"net/http/httptest"
+	"os"
+	"testing"
+
+	"github.com/ethereum/go-ethereum/swarm/api"
+	httpapi "github.com/ethereum/go-ethereum/swarm/api/http"
+	"github.com/ethereum/go-ethereum/swarm/storage"
+)
+
+func NewTestSwarmServer(t *testing.T) *TestSwarmServer {
+	dir, err := ioutil.TempDir("", "swarm-storage-test")
+	if err != nil {
+		t.Fatal(err)
+	}
+	storeparams := &storage.StoreParams{
+		ChunkDbPath:   dir,
+		DbCapacity:    5000000,
+		CacheCapacity: 5000,
+		Radius:        0,
+	}
+	localStore, err := storage.NewLocalStore(storage.MakeHashFunc("SHA3"), storeparams)
+	if err != nil {
+		os.RemoveAll(dir)
+		t.Fatal(err)
+	}
+	chunker := storage.NewTreeChunker(storage.NewChunkerParams())
+	dpa := &storage.DPA{
+		Chunker:    chunker,
+		ChunkStore: localStore,
+	}
+	dpa.Start()
+	a := api.NewApi(dpa, nil)
+	srv := httptest.NewServer(httpapi.NewServer(a))
+	return &TestSwarmServer{
+		Server: srv,
+		Dpa:    dpa,
+		dir:    dir,
+	}
+}
+
+type TestSwarmServer struct {
+	*httptest.Server
+
+	Dpa *storage.DPA
+	dir string
+}
+
+func (t *TestSwarmServer) Close() {
+	t.Server.Close()
+	t.Dpa.Stop()
+	os.RemoveAll(t.dir)
+}