[bug] Forest doesn't report forks correctly

# Summary

We expose changes to the current HEAD through the `ChainNotify` RPC call. Listeners on this method should be told about new blocks and about forks in the chain.

In Lotus, this is done through three types of messages: `revert,` `apply,` and `current.` When a new tipset is appended to the blockchain, the `apply` message is sent to listeners. If a longer branch is detected (a fork), Lotus reverts tipsets until the common root and then applies the tipsets from the new branch.

https://github.com/filecoin-project/lotus/blob/71927361046f9a11a024d605f71321224b0497b7/chain/store/store.go#L283-L287
```go
const (
HCRevert = "revert"
HCApply = "apply"
HCCurrent = "current"
)
```

In Forest, we do not do this correctly. In fact, we only send out the `apply` message. When a new heaviest tipset is selected, we pretend to immediately apply it:

https://github.com/ChainSafe/forest/blob/00c52a4c75d3387c9d5d1ec5ea1310c434b1e26d/src/chain/store/chain_store.rs#L51-L53

We need to have proper support for navigating the blockchain graph.

# Requirements

- [ ] Basic support for `revert` messages.
- [ ] Re-work state-machine internals to navigate between branches rather than directly jumping to the heaviest tipset.
- [ ] Figure out how to unit test this behavior.
- [ ] Figure out how to test this behavior with a devnet.

Note: The easiest solution may be to keep track of the last reported tipset and then intersperse additional `revert`s or `apply`s if required. This way, we don't have to re-design the internals of our state-machine.

# Motivation

Proper handling of forks is essential for Curio integration. Without it, we cannot mine successfully with Forest.