Chaining txs.

[LeilaWang]

https://hackmd.io/G0uD0r52Qca89BCINuDByQ

Tasks

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1. Aggregate pending_read_requests in the private kernel circuits and verify them in the base rollup circuit.
2. Include pending notes when selecting notes for the get_notes oracle call. If "read", add the note hash to pending_read_requests.
3. Create a new PXE api to add pending note to the db.
4. Add pending note data to the database when successfully submitting a tx.
5. Monitor the tx and clear pending note data if the tx is dropped.
6. Include chained txs in the rollup in the correct order.
7. Discard a tx if one of its backward links is not a settled or pending note hash.

[dan-aztec]

#2869 addresses first task (minus verification, since we are about to migrate the cpp circuits to noir)