Inspect mempool before submitting the same transaction again

[pdyraga]

In some places of ECDSA client there is a risk of submitting the same transaction (but with an increased nonce) again if the original one was not mined in a reasonable time frame. One example is the submit redemption signature monitoring loop, the other example is monitor and update operator status in a sortition pool loop. This situation usually happens when MaxGasPrice in the client configuration is set to a too low value and the client does not bump up the original transaction gas price what leads to keeping this transaction in a mempool for a long time.

We could have some logic making sure that if the same transaction is already in a mempool, the client is not trying to submit it again. This requires thorough research and design because it may happen that for some cases duplicate transactions are desirable.