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/keyring.rs

@@ -32,7 +32,7 @@ impl<'a> Keyring<'a> {
         &mut self,
         context: &Context,
         self_addr: *const libc::c_char,
-        sql: &dc_sqlite3_t,
+        sql: &SQLite,
     ) -> bool {
         // Can we prevent keyring and self_addr to be null?
         if self_addr.is_null() {