feat: service-based deferred listeners

[weierophinney]

In playing some more with Swoole task workers, I discovered that any
listeners you provide MUST be serializable - which poses a problem for
listeners that compose things such as HTTP clients, database adapters,
etc.

This patch:

• Adds a Psr\Container\ContainerInterface $container argument to
  TaskInterface::__invoke().
• Modifies TaskWorker to compose the PSR-11 container, and pass it to
  tasks as it invokes them.
• Adds ServiceBasedTask, which accepts a service name and payload
  arguments to its constructor. When invoked, it will pull the service
  from the container. If the service is a DeferredServiceListener, it
  will pull the listener from that instance and use that. It invokes the
  service.
• Adds DeferredServiceListener, which creates a ServiceBasedTask with
  the service name passed to its constructor, and defines a
  getListener() method for getting the actual listener to use when
  handling the task.
• Adds DeferredServiceListenerDelegator, which creates a
  DeferredServiceListener using the service name and the result of
  calling the delegator's factory argument.

What this means is that users can assign the
DeferredServiceListenerDelegator as a delegator factory on listeners
that should defer execution to the Swoole task workers. This will be
safer than the DeferredListenerDelegator in almost every case.

Since this patch changes the signature of an interface method, I'm declaring it a new major version feature.