add leveldb store

1. switch to viper for filer store configuration
2. simplify FindEntry() return values, removing “found”
3. add leveldb store

weed/filer2/entry.go

@@ -28,15 +28,14 @@ type Entry struct {
 	Chunks []*filer_pb.FileChunk `json:"chunks,omitempty"`
 }
 
-func (entry Entry) Size() uint64 {
+func (entry *Entry) Size() uint64 {
 	return TotalSize(entry.Chunks)
 }
 
-func (entry Entry) Timestamp() time.Time {
+func (entry *Entry) Timestamp() time.Time {
 	if entry.IsDirectory() {
 		return entry.Crtime
 	} else {
 		return entry.Mtime
 	}
 }
-