Use a fresh context to remove the temporary filter installed solely to
get the actor events. This should reduce chances of failure in a case
where the original context may be expired/cancelled.
Refactor removal into a `defer` statement for a more readable, concise
return statement.
Use BlockDelay as the window for receiving events on the SubscribeActorEvents
channel. We expect the user to have received the initial batch of historical
events (if any) in one block's time. For real-time events we expect them to
not fall behind by roughly one block's time.
