Keep GPBFT disabled until the node actually starts participating.

[Stebalien]

Until some storageminer process calls F3Participate, we should passively follow F3 via the finality certificate exchange instead of via pubsub to avoid wasting bandwidth. This way:

1. Pure clients can save a lot of bandwidth.
2. SPs can save bandwidth by delegating all F3 operations to a single client.

[masih]

This would probably touch some of #588? Would be nice to knock that one out too.

[Stebalien]

It should be mostly orthogonal, but I agree.

[Stebalien]

Two options:

1. Abstract over this in lotus and don't start the inner F3 module until we need it. That'll lead to a cleaner solution in this repo, but we'll also have some duplicate setup code (need a certstore, certserve, certsub, etc.).
2. Change F3.MessagesToSign to something like F3.Participate(), lazily starting the runner on-demand. Honestly, we don't really need a way to stop participating.

Personally, I'm leaning towards option 2. It's a bit of work in this repo, but not much.

[masih]

Option 2 sgtm too.