Batch insertion

[recmo]

Requires #258.

Modify the transaction worker service (B). From it's #258 behaviour:

1. It takes an item from the work-queue.
2. It flags it as in-progress.
3. It creates and signs an Ethereum transaction.
4. It waits for the transaction to be mined.
5. Marks the work item as done.

Into the following:

---

1. Check the work-queue. If it has more than MAX_BATCH unprocessed items or the oldest unprocessed item is older than MAX_DELAY, proceed.
2. Take up to MAX_BATCH unprocessed items from work queue, oldest first.
3. Mark these as 'processing' with a timestamp.
4. Request next available nonce for the transaction, and flag this nonce as taken.
5. Send the batch to a prover.
6. When the proof is done, create and sign a transaction.
7. Send transaction to mempool using one or more API endpoints.
8. Babysit the transaction, re-send and/or raise gasprice if necessary.
9. Once the transaction is mined and sufficiently confirmed, mark work items as done.

TBD:

• Create a ZKP prover services for the compute-heavy proving tasks
• Mechanism for recovering from failed work. For example a timeout based on the timestamp in 3.
• Do we want to split transaction signing/sending to a separate service? It can be a reusable component.

[kustosz]

Re: "ZKP prover services for the compute-heavy proving tasks" – this is now tracked in worldcoin/semaphore-mtb#3