Include mempool stats in block-producer's status

[Mirko-von-Leipzig]

Include some mempool metrics in the block producer's status endpoint.

This could include:

• number of inflight transactions, both
  • unbatched and
  • in total (including in batches)
• number of inflight batches
• ..maybe note counts? But this is maybe less important

The status endpoint must not lock the mempool to obtain this information as this is an externally accessible endpoint. Instead the mempool can emit these stats using a watch channel.

[Mirko-von-Leipzig]

This is blocked by the ongoing work in #1234 and its follow-up work.