* txmgr: Queue.Send() uses q.txMgr.SendAsync

This should ensure that transactions are confirmed on chain in the order Queue.Send() is called, without sacrificing parallel tx submission.

* implement SendAsync in op-challenger test stubTxMgr

It doesn't preserve the nonces like the production txMgr does.

* TrySend also ensures synchronous tx nonce ordering (on success)

* factor out HandleResponse fn

* remove unused code

* unexport handler