[block-producer]: follow-up tasks

[Mirko-von-Leipzig]

Follow-up tasks from #515. Some of these require work in miden-base.

• [ ] Create and use ProposedBatch in miden-base (source)
• [ ] Use a more correct BatchId (source)
• [ ] Create and use ProposedBlock in miden-base (source)
• [ ] Loosen block selection strategy (source)
• [ ] #523
• [x] Improve block/batch simulation times (source and source)
• [ ] Amend GetTransactionInputs to found instead of missing notes (source)
• [ ] Instrument mempool
• [x] Wrap AuthenticatedTransaction::inner in an Arc for cheaper copies.
• [ ] Move graph tests into a separate tests.rs file.
• [ ] Move batch limits from TransactionBatch into the mempool's batch selection strategy
• [ ] Move block limits from the block builder into the mempool's block selection strategy
• [ ] Remove transactions from the error variant of batch builder error and in general revisit all errors there.

[bobbinth]

Nice! I added this issue to the current milestone, though probably not everything will end up there (e.g., ProposedBatch and ProposedBlock is definitely for the next milestone).