Change OdbBackend.ReadPrefix() signature

**From**

```
public abstract int ReadPrefix(
  byte[] shortOid,
  int prefixLen,
  out byte[] oid,
  out Stream data,
  out ObjectType objectType);
```

This should ease (a little bit) the task of implementing a backend

**To**

```
public abstract int ReadPrefix(
  string shortSha,
  out ObjectId id,
  out Stream data,
  out ObjectType objectType);
```