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)
[IMAPx] Deleted messages not always saved to the server properly

This is an outcome of investigation of bug 734300, but it probably
does not fix that bug as such, only something similar.