[WIP] fix builds against sqlite 3.8.x

This seems to fix the build against SQLite 3.8.x versions
when using a Debug build.

This is just a proof of concept, since I don't
really know what I'm doing with SQLite or HHVM's code.

Is there any value in using RepoTxnQuery for PRAGMAs though?

It seems that SQLite doesn't like reading the value of journal_mode
in RepoTxnQuery, immediately after setting it.

The documentation for journal_mode only says:

"Note also that the journal_mode cannot be changed while a transaction is active."
http://www.sqlite.org/pragma.html#pragma_journal_mode