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.

src/dc_move.rs

@@ -15,7 +15,7 @@ pub unsafe fn dc_do_heuristics_moves(
     let stmt: *mut sqlite3_stmt = 0 as *mut sqlite3_stmt;
     if !(dc_sqlite3_get_config_int(
         context,
-        &context.sql.clone().read().unwrap(),
+        &context.sql,
         b"mvbox_move\x00" as *const u8 as *const libc::c_char,
         1i32,
     ) == 0i32)