Optimize pack readHeader() implementation

Load pack header length and 15 header entries with single backend
request. This eliminates separate header Load() request for most pack
files and significantly improves index.New() performance.

Signed-off-by: Igor Fedorenko <igor@ifedorenko.com>

internal/pack/pack_internal_test.go

@@ -0,0 +1,46 @@
+package pack
+
+import (
+	"bytes"
+	"encoding/binary"
+	"io"
+	"testing"
+
+	"github.com/restic/restic/internal/crypto"
+	rtest "github.com/restic/restic/internal/test"
+)
+
+type countingReaderAt struct {
+	delegate        io.ReaderAt
+	invocationCount int
+}
+
+func (rd *countingReaderAt) ReadAt(p []byte, off int64) (n int, err error) {
+	rd.invocationCount++
+	return rd.delegate.ReadAt(p, off)
+}
+
+func TestReadHeaderEagerLoad(t *testing.T) {
+
+	testReadHeader := func(entryCount uint, expectedReadInvocationCount int) {
+		expectedHeader := rtest.Random(0, int(entryCount*entrySize)+crypto.Extension)
+
+		buf := &bytes.Buffer{}
+		buf.Write(rtest.Random(0, 100))                                     // pack blobs data
+		buf.Write(expectedHeader)                                           // pack header
+		binary.Write(buf, binary.LittleEndian, uint32(len(expectedHeader))) // pack header length
+
+		rd := &countingReaderAt{delegate: bytes.NewReader(buf.Bytes())}
+
+		header, err := readHeader(rd, int64(buf.Len()))
+		rtest.OK(t, err)
+
+		rtest.Equals(t, expectedHeader, header)
+		rtest.Equals(t, expectedReadInvocationCount, rd.invocationCount)
+	}
+
+	testReadHeader(1, 1)
+	testReadHeader(eagerEntries-1, 1)
+	testReadHeader(eagerEntries, 1)
+	testReadHeader(eagerEntries+1, 2)
+}