[Refactor] Remove BFT channels

[kaimast]

The following changes are included in #4039, but I moved them to a separate PR so they are easier to review.

snarkOS relies on channels for communication between modules in the consensus and bft crates. This indirection exists mostly to avoid circular dependencies between modules, but has the following drawbacks.

• The code is generally harder to read as, instead of a simple function call, it relies on two channels: one to send the call and one to return the result to.
• A single operation now involves multiple tokio tasks, as the receiving side of each channel has a distinct task waiting on it.
• Error handling is more complicated, because panics do not propagate through channels easily.
• Channels can return errors, e.g., while waiting on a oneshot::Receiver, that we then need to handle in the code.
• Waiting on a channel is an async task and, thus, requires await.

The last point, especially, makes it harder to improve locking/coordination between different tasks as regular locks cannot be held across awaits and blocking tasks cannot call async functions. The former can be solved through async locks, but those are generally less efficient regular locks. The latter is harder to solve, as we use blocking tasks in multiple places while advancing the ledger.

This PR removes BFTSender and BFTReceiver, but leaves other channels in place. The BFT channels are replaced by two callback traits, one for the Primary and one for Sync. Instead of waiting on BFTReceiver the BFT struct now implements those to traits and registers itself as a callback for each.