Do not create the wallet struct directly; instead, call new. (#707)

The bug comes with the SQLx-sqlite pool bug, where several connections are
created by default, but the `new` function takes care of that, fixing that bug
by making a single instance of the database.

If constructed directly, the pool would create several connections to the
database, which in most instances is fine, but with SQLite :memory: each
connection is entirely independent.

Also follow documentation to make sure that failed `acquire` will not end up
dropping connections by setting  test_before_acquire to false

     However, if your workload is sensitive to dropped connections such as using an in-memory
     SQLite database with a pool size of 1, you can pretty easily ensure that a cancelled
     `acquire()` call will never drop connections by tweaking your [`PoolOptions`]:

     * Set [`test_before_acquire(false)`][PoolOptions::test_before_acquire]
     * Never set [`before_acquire`][PoolOptions::before_acquire] or
       [`after_connect`][PoolOptions::after_connect].

crates/cdk-cli/src/main.rs

@@ -128,8 +128,6 @@ async fn main() -> Result<()> {
                     }
                 };
 
-                sql.migrate().await;
-
                 Arc::new(sql)
             }
             "redb" => {