Support sequential consistency in SQLite implementation (#64)

This is a bit tricky because the `Storage` trait is `Send + Sync`, but
an SQLite connection is neither. Since this is not intended for
large-scale use, the simple solution is just to open a new SQLite
connection for each transaction. More complex alternatives include
thread-local connection pools or a "worker thread" that owns the
connection and communicates with other threads via channels.

This also updates the InMemoryStorage implementation to be a bit more
strict about transactional integrity, which led to a number of test
changes.

core/src/server.rs

@@ -307,7 +307,12 @@ mod test {
     {
         let _ = env_logger::builder().is_test(true).try_init();
         let storage = InMemoryStorage::new();
-        let res = init(storage.txn()?.as_mut())?;
+        let res;
+        {
+            let mut txn = storage.txn()?;
+            res = init(txn.as_mut())?;
+            txn.commit()?;
+        }
         Ok((Server::new(ServerConfig::default(), storage), res))
     }