completeExceptionally(Throwable) must be called from synchronized block since we must not complete
the future (exceptionally) when we're accessing the context - if there was an exception and we would retry,
the context could be accessed concurrently by dangling handlers and retry execution (that does not synchronize
on the same future).
When completeExceptionally executes before the other responses processing, the future will be marked as done
and as soon as the other responses get into the synchronized block, these will check isDone and return.
If the response is being processed in sync block, running completeExceptionally and the related callbacks
will be blocked until we finish the processing.
Nested Class Summary
Nested classes/interfaces inherited from class java.util.concurrent.CompletableFuture