Bug 735523 - Allow EBookSqlite to be used without transactions

If we use it without grouping changes into transactions by using
e_book_sqlite_lock(), and if we have a cancellation in the calls to
actually make the changes, then this warning triggers because the
new cancellation doesn't match the NULL that we have stored for the
non-existent overall transaction.

Yes, we probably ought to be using locking in EWS but we're not. It's
perfectly valid to make changes without a transaction lock. Shut up
about it.

(cherry picked from commit 629a98e292b9e4cb09408663af37477dc1a93f61)