[Stability] Fix GC for tasks

[rob-maron]

This PR:

Addresses a slow memory leak where we are not removing tasks from the task map. It does this by:

• adding a new type TaskMap that is basically a type alias for what we had
• impls a function prune_tasks that takes a generator for the event and the view number and cleans up
• uses this function for every task map

This PR does not:

Fix the entirety of the memory leak issue.

Key places to review:

web_server_network.rs

I have tested that it cleans up all old tasks ✅