Refactor the internal sql interface somewhat

Experiment with refactoring the internal sql interface a bit.  My
original goal was to modify the schema and thus refactor to a state
where it would be sane to write tests for dc_sqlite_open() (and/or
however it ends up refactored) to assert schame changes before/after.

tests/stress.rs

@@ -1,7 +1,7 @@
 //! Stress some functions for testing; if used as a lib, this file is obsolete.
 
 use std::collections::HashSet;
-use std::ffi::{CStr, CString};
+use std::ffi::CString;
 
 use mmime::mailimf_types::*;
 use tempfile::{tempdir, TempDir};