feat: Sequencers should bound execution time

[PhilWindle]

Currently, sequencers work by retrieving transactions from the mempool, up to a configured limit. They execute these transactions and then attempt to gain attestations. Attesting requires re-executing the transactions.

The sequencer pays no consideration to the amount of time it takes to execute the transactions it retrieves, so if it takes too long then it misses the slot.

The sequencer should bound execution time based on the configured time table, only starting transaction execution if there is greater than some proportion of the available block-building time available. The sequencer should then request attestations on whatever they have managed to build.

This should ensure we maintain a stream of blocks at the expense of trying to achieve a specific TPS.

[just-mitch]

It would be nice we had a way to estimate mana usage by transaction before it was processed. Then the sequencer could keep a running tab of its mana/s and then more effectively binpack its block based on the time remaining.

[just-mitch]

But yes, at least cutting it off when it is approaching the boundary of the timetable is a good first step.