[DRB] - Start the DRB calculation two epochs in advance

[shenkeyao]

Closes #3857.

This PR:

• Calls the DRB function in the vote task to start the computation for two views in advance.
• Updates the vote task type to include spawned DRB computation tasks.
• Moves and updates the DRB implementation file.

This PR does not:

• Handles the use of the DRB result, which will be done in the upcoming PRs.

Key places to review:

• All files, but the main changes are in handlers.rs.

[ss-es]

do we need a BTreeMap here? I think we only want to run one of these calculations at a time

we can e.g. store the seeds and results in a map and trigger the start of the calculation at block_height % epoch_height = 1 or something

[shenkeyao]

block_height % epoch_height = 1

Assuming this is (qc_block_number + 3) % task_state.epoch_height == 0 instead?

Makes sense not to use BTreeMap. I'm going to add a DrbTasks struct for this, containing the previous DRB result and the in-progress DRB calculation task, since we may need both of them in each epoch. Hopefully, it's not an overkill!

[shenkeyao]

On second thought, BTreeMap might be more suitable here than a HashMap or the new struct I proposed above. We may have at most three things to store here, two DRB results and one in-progress task. I think BTreeMap makes garbage collection easier because the three epochs may not be consecutive due to the membership.

Lmk if this doesn't make sense or there's a better design though!

[shenkeyao]

As discussed on Zulip, I'll update the maps in a separate PR. Issue: #3933.

[ss-es]

I think this looks good, but I left some thoughts about the BTreeMap on zulip