Add caching for pending proposals

[toninorair]

An approach that allows:

• fetching pending proposals once inside worker
• caching them
• reading this info from cache for all scenarios.

[toninorair]

So this assumes the slow part is getting all pending proposals? I tend to think its the running the migrations/executing them. Want to try on the mainnet proposal branch to benchmark?

About 2 min difference

[jflatow]

Closing as this is better addressed by #700