Update state machine to use a SectorSet AMT

[ZenGround0]

Description

After the work begun in #2887 has provided go-filecoin with an AMT-based SectorSet we need to update the miner actor to track its sector storage in this datastructure.

This will make #3007 obsolete.

Acceptance criteria

Use SectorSet instead of golang map in miner actor storage

Risks + pitfalls

Where to begin

Look at how HAMT based storage works. The AMT should be designed similarly enough that hooking up vm blockstorage to the AMT should be straightforward.

[anorth]

This issue is blocked by implementation of the AMT IPLD structure.

A spec for this is happening here: ipld/specs#137

[anorth]

@whyrusleeping has the initial implementation of AMT here: https://github.com/filecoin-project/go-amt-ipld

cc @hannahhoward while you are investigating HAMT storage layer improvements #3223 #3078. My first glance at the AMT code looks like it has the interface we need to swap in our own storage layer.

[anorth]

Done (@acruikshank)