Fix alignment in chunkedContentCoder

An unaligned atomic bug was unfortunately introduced in blevesearch#119 because the
`bytesWritten` field was placed at the end of the `chunkedContentCoder`
struct.

This places this after the bytes.Buffers and the bool causing it to be
misaligned.

The ideal placement of this variable is not entirely clear but placing
it before the progresiveWrite bool should help.

An alternative would be to just place this atomic field at the top of
the struct then there would be no risk of it becoming misaligned in
future.

I moved a few things around to reduce the size of the struct too but it
could be possible to adjust things a little more to make the struct a
little smaller.

Signed-off-by: Andrew Thornton <art27@cantab.net>

contentcoder.go

@@ -31,27 +31,26 @@ func init() {
 	reflectStaticSizeMetaData = int(reflect.TypeOf(md).Size())
 }
 
-var termSeparator byte = 0xff
-var termSeparatorSplitSlice = []byte{termSeparator}
+var (
+	termSeparator           byte = 0xff
+	termSeparatorSplitSlice      = []byte{termSeparator}
+)
 
 type chunkedContentCoder struct {
 	final     []byte
 	chunkSize uint64
 	currChunk uint64
 	chunkLens []uint64
 
+	compressed []byte // temp buf for snappy compression
+
 	w                io.Writer
+	bytesWritten     uint64 // atomic access to this variable
 	progressiveWrite bool
 
+	chunkMeta    []MetaData
 	chunkMetaBuf bytes.Buffer
 	chunkBuf     bytes.Buffer
-
-	chunkMeta []MetaData
-
-	compressed []byte // temp buf for snappy compression
-
-	// atomic access to this variable
-	bytesWritten uint64
 }
 
 // MetaData represents the data information inside a
@@ -64,7 +63,8 @@ type MetaData struct {
 // newChunkedContentCoder returns a new chunk content coder which
 // packs data into chunks based on the provided chunkSize
 func newChunkedContentCoder(chunkSize uint64, maxDocNum uint64,
-	w io.Writer, progressiveWrite bool) *chunkedContentCoder {
+	w io.Writer, progressiveWrite bool,
+) *chunkedContentCoder {
 	total := maxDocNum/chunkSize + 1
 	rv := &chunkedContentCoder{
 		chunkSize:        chunkSize,
@@ -191,7 +191,6 @@ func (c *chunkedContentCoder) Add(docNum uint64, vals []byte) error {
 //
 // | ..... data ..... | chunk offsets (varints)
 // | position of chunk offsets (uint64) | number of offsets (uint64) |
-//
 func (c *chunkedContentCoder) Write() (int, error) {
 	var tw int