Better support for WAL

```
Support sqlite3_wal_checkpoint and sqlite3_wal_checkpoint_v2 calls

http://sqlite.org/c3ref/wal_checkpoint.html
http://sqlite.org/c3ref/wal_checkpoint_v2.html
```

Original issue reported on code.google.com by `ser...@gmail.com` on 1 Aug 2011 at 5:27
