test(beacon_chain): add tests for concurrent stop and nil wallclock scenarios

Adds a `Stop` method to the `EthereumBeaconChain` to allow for graceful shutdown of the background goroutines. This prevents goroutine leaks when the beacon chain is no longer needed.

Adds tests to ensure that calling `Stop` concurrently with callback registration does not cause a race condition or panic.

Adds a test to specifically address a production issue where calling `OnEpochChanged` on a nil `EthereumBeaconChain` receiver caused a panic. This test simulates the scenario where the wallclock becomes nil and verifies that the expected panic occurs, highlighting the need for nil checks before calling methods on the wallclock.

refactor(beacon-chain): potential race to write callbacks