Better support for WAL

[Igor Sereda]

Originally reported on Google Code with ID 33 ``` 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 ```

Reported by `sereda` on 2011-08-01 17:27:28