Journal single item batch optimization

The journal is a list of entries e.g.

```
Start(3)
  BatchItem(K V ...)
  BatchItem(K V ...)
  BatchItem(K V ...)
End(Checksum)
```

For non-batch small value writes, this format is unnecessarily large; instead, single item writes could be written using a single (new) entry type, e.g.

```
Item(Checksum, K, V ...)
```