Rewrite the backup procedure to reduce memory usage

Main changes:

* Change the listing order of files/directories so that the local and remote
  snapshots can be compared on-the-fly.

* Introduce a new struct called EntryList that maintains a list of
  files/directories, which are kept in memory when the number is lower, and
  serialized into a file when there are too many.

* EntryList can also be turned into an on-disk incomplete snapshot quickly,
  to support fast-resume on next run.

* ChunkOperator can now download and upload chunks, thus replacing original
  ChunkDownloader and ChunkUploader.  The new ChunkDownloader is only used
  to prefetch chunks during the restore operation.

src/duplicacy_utils.go

@@ -14,6 +14,7 @@ import (
 	"strconv"
 	"strings"
 	"time"
+	"runtime"
 
 	"github.com/gilbertchen/gopass"
 	"golang.org/x/crypto/pbkdf2"
@@ -460,3 +461,16 @@ func AtoSize(sizeString string) int {
 
 	return size
 }
+
+func PrintMemoryUsage() {
+
+	for {
+		var m runtime.MemStats
+		runtime.ReadMemStats(&m)
+
+		LOG_INFO("MEMORY_STATS", "Currently allocated: %s, total allocated: %s, system memory: %s, number of GCs: %d",
+		         PrettySize(int64(m.Alloc)), PrettySize(int64(m.TotalAlloc)), PrettySize(int64(m.Sys)), m.NumGC)
+
+		time.Sleep(time.Second)
+	}
+}