valblk: fix Writer.Size() performance

`Writer.Size()` unnecessarily loops through all the blocks instead of
using `totalBlockBytes`. This seems like an oversight: we only append
to `blocks` in `compressAndFlush` where we also update
`totalBlockBytes`. I verified that in all Pebble tests these values are
the same, using `if sz != w.totalBlockBytes { panic() }`.

```
name                                                                                                        old time/op    new time/op     delta
WriterWithVersions/vals-per-key=1/format=(Pebble,v5)/block=32KB/filter=false/compression=NoCompression        77.2ms ± 1%     76.6ms ± 0%    -0.70%  (p=0.008 n=5+5)
WriterWithVersions/vals-per-key=10/format=(Pebble,v5)/block=32KB/filter=false/compression=NoCompression        752ms ± 0%       99ms ± 1%   -86.83%  (p=0.008 n=5+5)
WriterWithVersions/vals-per-key=10000/format=(Pebble,v5)/block=32KB/filter=false/compression=NoCompression     908ms ± 0%      103ms ± 0%   -88.64%  (p=0.008 n=5+5)

name                                                                                                        old speed      new speed       delta
WriterWithVersions/vals-per-key=1/format=(Pebble,v5)/block=32KB/filter=false/compression=NoCompression       462MB/s ± 1%    466MB/s ± 0%    +0.70%  (p=0.008 n=5+5)
WriterWithVersions/vals-per-key=10/format=(Pebble,v5)/block=32KB/filter=false/compression=NoCompression     53.0MB/s ± 0%  402.7MB/s ± 1%  +659.47%  (p=0.008 n=5+5)
WriterWithVersions/vals-per-key=10000/format=(Pebble,v5)/block=32KB/filter=false/compression=NoCompression  49.1MB/s ± 0%  431.8MB/s ± 0%  +780.16%  (p=0.008 n=5+5)
```

sstable/valblk/writer.go

@@ -161,10 +161,7 @@ func (w *Writer) AddValue(v []byte) (Handle, error) {
 
 // Size returns the total size of currently buffered value blocks.
 func (w *Writer) Size() uint64 {
-	var sz uint64
-	for _, blk := range w.blocks {
-		sz += uint64(blk.block.LengthWithTrailer())
-	}
+	sz := w.totalBlockBytes
 	if w.buf != nil {
 		sz += uint64(len(w.buf.b))
 	}