[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)
• [block-producer]: missed block strategy #523
• Improve block/batch simulation times (source and source)
• Amend GetTransactionInputs to found instead of missing notes (source)
• Instrument mempool
• Wrap AuthenticatedTransaction::inner in an Arc for cheaper copies.

[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).