Support Epoch Extensions in HotStuff Committee

[jordanschalm]

This PR updates the Consensus Committee to support epoch extensions. Addresses #5730.

Changes

• Modifies protocol.Epoch FinalView method to account for any extensions.
• Modifies committees.Consensus to handle EpochExtended protocol event. When we observe an epoch extension, we simply re-compute the leader selection for the entire epoch, including the new view range added by the extension. This is slightly wasteful, incurring a cost of around 20-30ms per extension on Mainnet, but simplifies the business logic and state quite a bit compared to tracking each extension independently.
• Moves internal event processing to a shared channel Now that we can move into and out of EFM several times over the lifecycle of the component, it is easier to reason about correctness if we use a shared channel for events to enforce in-order processing. The event consumer function prepares a (potentially blocking) event handler function to be executed and passes the closure into the channel. The worker goroutine pulls event handler closures off the channel and executes them in order.