support WAL

[c-cube]

looking at https://sqlite.org/wal.html :

• a command to execute pragmas could be useful (althoug exec does the job)
• triggering checkpoints by hand can be useful using https://sqlite.org/c3ref/wal_checkpoint_v2.html

[mmottl]

Please feel free to submit a PR for any solutions, or close the issue if this is not relevant anymore.