Transactionpool: Make `ready_at` return earlier (#8995)

`ready_at` returns when we have processed the requested block. However,
on startup we already have processed the best block and there
are no transactions in the pool on startup anyway. So, we can set `updated_at`
to the best block on startup.

Besides that `ready_at` now returns early when there are no ready nor
any future transactions in the pool.