FIP 34 - Fix pre-commit deposit independent of sector content

[lemmih]

Issue summary

Checklist:

• Are there any changes to the actor data structures that we need to be aware of?
• Are there any changes to the network protocol?
• Are there any changes to node behavior (eg. changes in block validation criteria, random number generation, etc)?
• Are there any new magic numbers?

Other information and links

https://github.com/filecoin-project/FIPs/blob/master/FIPS/fip-0034.md

[LesnyRumcajs]

Data structures change

SectorPreCommitOnChainInfo should be changed to what builtin-actors expose. Same applies to SectorPreCommitInfo. We do some serde tricks so it may not be possible to import it, most likely we'll just need adapt the changes in our implementation.

Logical flow
It should be self-contained in builtin-actors.

Related PR: filecoin-project/builtin-actors#484

⚠️ Tests ⚠️
Those structs are used only in our RPC API which is not covered by tests. It would be great to at least test that it works manually.