Venus: Miner actor SectorCommitments map should be a lookup

[ZenGround0]

Description

All other map datastructures within actor state are implemented as hamt-based lookups. From conversation with @acruikshank it looks like the SectorCommitments implementation was meant to be moved to a lookup but it slipped through the cracks.

Acceptance criteria

We have evaluated whether this is the right thing to do.

We have changed the implementation of the SectorCommitments map to use a lookup.

Risks + pitfalls

cc @laser in case this issue is missing key context suggesting we not use a lookup to implement this map.

This change can't be introduced live into a new network without a model for change protocol upgrade (protocol breaking)

Where to begin

Understand actor lookups and modify miner.go

[anorth]

@ZenGround0 FYI this issue might be either redundant or repurposed into your SectorSet(/Map) work

[anorth]

I think this is the same as #2888