Etherlink: strengthen the transaction pool for the sequencer

This is to limit the possibility of it being stuck because running out resources (mainly memory).

Sequencer

• !12666 (merged) Bound the blueprint size. @picdc
  • Not about the transaction pool, but also bound the block size on the kernel side, to prevent it from going OutOfTicks when collecting the chunks and decoding them, and thus rejecting them while the sequencer happily applied it. @picdc

Transaction Pool

• !12749 (merged) Bound the size of the transaction pool itself (both the number of addresses, and the number of transactions by address). @rodibozman
• !12741 (merged) Bound the time a transaction can live in the pool, i.e. removing it passed a timeout. @rodibozman
• !12834 (merged) Bound the size of a transaction. @rodibozman

UX & Tests

• !12873 (merged) Add the txpool_content RPC. @hantang.sun