Use external pubsub

[hannahhoward]

Goals

Reduce copy-pasta around setting up subscriptions, avoid accidental mutex errors with subscription lists

Implementation

• use a little library https://github.com/hannahhoward/go-pubsub that implements this functionality in a generic way
• remove non-specific logic from all the hooks.
• also using this pattern in go-data-transfer, go-fil-markets in filecoin ecosystem, repo can move into PL namespace if someone is willing to own it (it's not functionality that is particularly IPFS-ey or Filecoin-ey)