update statediff algorithm for wavesrv / remote instances (#530)

* remote statemap from waveshell server (diff against initial state)

* move ShellStatePtr from sstore to packet so it can be passed over the wire

* add finalstatebaseptr to cmddone

* much improved diff computation code on wavesrv side

* fix displayname -- now using hash

* add comments, change a couple msh.WriteToPtyBuffer calls to log.Printfs

waveshell/pkg/statediff/linediff.go

@@ -186,6 +186,9 @@ func readEncodedStringArray(buf *bytes.Buffer) ([]string, error) {
 
 func (rtn *LineDiffType) Decode(diffBytes []byte) error {
 	rtn.Clear()
+	if len(diffBytes) == 0 {
+		return nil
+	}
 	r := bytes.NewBuffer(diffBytes)
 	version, err := binary.ReadUvarint(r)
 	if err != nil {