Transaction pool: Remove futures-diagnose and thread pool (#9074)

* Transaction pool: Remove futures-diagnose and thread pool

This pr removes `futures-diagnose` as this isn't used anymore. Besides
that the pr also removes the thread pool that was used to validate the
transactions in the background. Instead of this thread pool we now spawn
two separate long running tasks that we use to validate the
transactions. All tasks of the transaction pool are now also spawned as
essential tasks. This means, if any of these tasks is stopping, the node
will stop as well.

* Update client/transaction-pool/src/api.rs