Re-use zlib writers to reduce memory usage

benchcmp:

    benchmark                       old ns/op       new ns/op       delta
    BenchmarkArchiveDirectory-4     29624960475     29511001504     -0.38%
    BenchmarkSaveJSON-4             379833          225609          -40.60%

    benchmark                       old allocs     new allocs     delta
    BenchmarkArchiveDirectory-4     546736         540642         -1.11%
    BenchmarkSaveJSON-4             150            126            -16.00%

    benchmark                       old bytes      new bytes     delta
    BenchmarkArchiveDirectory-4     1372476952     438519336     -68.05%
    BenchmarkSaveJSON-4             1462773        9087          -99.38%

server_test.go

@@ -0,0 +1,68 @@
+package restic_test
+
+import (
+	"bytes"
+	"crypto/sha256"
+	"encoding/json"
+	"testing"
+
+	"github.com/restic/restic"
+	"github.com/restic/restic/backend"
+)
+
+type testJSONStruct struct {
+	Foo uint32
+	Bar string
+	Baz []byte
+}
+
+var serverTests = []testJSONStruct{
+	testJSONStruct{Foo: 23, Bar: "Teststring", Baz: []byte("xx")},
+}
+
+func TestSaveJSON(t *testing.T) {
+	be := setupBackend(t)
+	defer teardownBackend(t, be)
+	key := setupKey(t, be, "geheim")
+	server := restic.NewServerWithKey(be, key)
+
+	for _, obj := range serverTests {
+		data, err := json.Marshal(obj)
+		ok(t, err)
+		data = append(data, '\n')
+		h := sha256.Sum256(data)
+
+		blob, err := server.SaveJSON(backend.Tree, obj)
+		ok(t, err)
+
+		assert(t, bytes.Equal(h[:], blob.ID),
+			"TestSaveJSON: wrong plaintext ID: expected %02x, got %02x",
+			h, blob.ID)
+	}
+}
+
+func BenchmarkSaveJSON(t *testing.B) {
+	be := setupBackend(t)
+	defer teardownBackend(t, be)
+	key := setupKey(t, be, "geheim")
+	server := restic.NewServerWithKey(be, key)
+
+	t.ResetTimer()
+	//t.SetBytes(int64(size))
+
+	obj := serverTests[0]
+
+	data, err := json.Marshal(obj)
+	ok(t, err)
+	data = append(data, '\n')
+	h := sha256.Sum256(data)
+
+	for i := 0; i < t.N; i++ {
+		blob, err := server.SaveJSON(backend.Tree, obj)
+		ok(t, err)
+
+		assert(t, bytes.Equal(h[:], blob.ID),
+			"TestSaveJSON: wrong plaintext ID: expected %02x, got %02x",
+			h, blob.ID)
+	}
+}