Migrations: Drop cache after some period

[arajasek]

Checklist

• This is not a new feature or an enhancement to the Filecoin protocol. If it is, please open an FIP issue.
• This is not a new feature request. If it is, please file a feature request instead.
• This is not brainstorming ideas. If you have an idea you'd like to discuss, please open a new discussion on the lotus forum and select the category as Ideas.
• I have a specific, actionable, and well motivated improvement to propose.

Lotus component

• lotus daemon - chain sync
• lotus miner - mining and block production
• lotus miner/worker - sealing
• lotus miner - proving(WindowPoSt)
• lotus miner/market - storage deal
• lotus miner/market - retrieval deal
• lotus miner/market - data transfer
• lotus client
• lotus JSON-RPC API
• lotus message management (mpool)
• Other

Improvement Suggestion

Currently, Lotus does not drop the migration cache upon a successful migration. This is intentional, as a minor reorg around the upgrade could cause us to need to rerun the migration (and we want the cache for that).

We should eventually drop this cache, though -- as far as I can tell, that only happens on node restart right now. Some heuristic like "made it 100 epochs after the migration, let's drop" might be good, though suggested approaches are welcome.

[arajasek]

@travisperson's research showed this cache is actually not that large. It's not worth dropping.