rework size estimation & dry sends

- use control connection (gRPC)
- use uint64 everywhere => fixes https://github.com/zrepl/zrepl/issues/463
- [BREAK] bump protocol version

closes https://github.com/zrepl/zrepl/pull/518
fixes https://github.com/zrepl/zrepl/issues/463

util/bytecounter/bytecounter_readcloser.go

@@ -9,7 +9,7 @@ import (
 // its interface and counting the bytes written to during copying.
 type ReadCloser interface {
 	io.ReadCloser
-	Count() int64
+	Count() uint64
 }
 
 // NewReadCloser wraps rc.
@@ -19,11 +19,11 @@ func NewReadCloser(rc io.ReadCloser) ReadCloser {
 
 type readCloser struct {
 	rc    io.ReadCloser
-	count int64
+	count uint64
 }
 
-func (r *readCloser) Count() int64 {
-	return atomic.LoadInt64(&r.count)
+func (r *readCloser) Count() uint64 {
+	return atomic.LoadUint64(&r.count)
 }
 
 var _ io.ReadCloser = &readCloser{}
@@ -34,6 +34,9 @@ func (r *readCloser) Close() error {
 
 func (r *readCloser) Read(p []byte) (int, error) {
 	n, err := r.rc.Read(p)
-	atomic.AddInt64(&r.count, int64(n))
+	if n < 0 {
+		panic("expecting n >= 0")
+	}
+	atomic.AddUint64(&r.count, uint64(n))
 	return n, err
 }

util/envconst/envconst.go

@@ -73,6 +73,24 @@ func Int64(varname string, def int64) (d int64) {
 	return d
 }
 
+func Uint64(varname string, def uint64) (d uint64) {
+	var err error
+	if v, ok := cache.Load(varname); ok {
+		return v.(uint64)
+	}
+	e := os.Getenv(varname)
+	if e == "" {
+		d = def
+	} else {
+		d, err = strconv.ParseUint(e, 10, 64)
+		if err != nil {
+			panic(err)
+		}
+	}
+	cache.Store(varname, d)
+	return d
+}
+
 func Bool(varname string, def bool) (d bool) {
 	var err error
 	if v, ok := cache.Load(varname); ok {