Support Parseq plan local cache to reduce fanout calls.

Sometimes application make duplicate downstream calls when serving a request. One way is to make the Task sharable and pass it around, but it’s sometimes hard to refactor the code while maintaining code readability. It would be great that parseq can provide some cache capability to avoid duplicate calls.

Some idea may be: ParSeq can provide a plan-local cache <K, Task> where K is user-defined key and Task computes result. It would make sense to implement it on a ParSeq level because:

1. ParSeq has a well defiled Plan life-cycle with hook when Plan completes (even after response has been sent to the client)
2. Natural API for asynchronous computation which is a most common use case for a cache (we have seen similar feature requests for Rest.li request results)

Start by tracing ParSeq's plan lifecycle and the completion hook described in the issue, then determine where a plan-local cache API would fit. Review how asynchronous Tasks are represented and shared, and define done as preventing duplicate downstream computations within one plan while handling plan completion and cleanup.