Minimize chances of missed withdrawals

[mjkoo]

Is your feature request related to a problem? Please describe. Right now we watch for transfer events using the subscribe rpc feature, we should more robustly handle any instances where we might miss transfers

Describe the solution you'd like In addition to eth_subscribe, consider one or both of:

• Periodic polling of event logs since last polled block, ensuring that all transfers that meet confirmation requirements were processed
• A way to manually trigger processing a transaction by providing a txhash, by which we could retrieve the event logs and check reqs / ensure processed

Describe alternatives you've considered In order to miss a transfer >1/3 of verifiers would have to miss it, we could increase number of verifiers to decrease this probability with no code changes.

[myhndl]

Spoke with @Rizato , this one is high prio, will move up in column.

[Rizato]

Wrapping this part in from #3 Need to retry failed approve_withdrawals as part of the recovery from missed transfers.

I'm unclear looking at the code what happens if a transaction times out, specifically in the case where a transaction has confirmed on the sidechain and funds are released on mainnet, but gas fees are too high and the tx doesn't confirm. This is mostly theoretical with a 20Gwei default fee, but could result in funds not actually being delivered.