[Narwhal] remove SharedCommittee and SharedWorkerCache

[mwtian]

When Narwhal needs to support epoch updates, data structures for Committee and WorkerCache can get updated dynamically. So SharedCommittee and SharedWorkerCache were created to support this use case.

Now Narwhal is recreated on epoch changes. The SharedCommittee and SharedWorkerCache wrappers are no longer needed, because the underlying Committee and WorkerCache structs never change throughput the lifetime of each Narwhal instance. Input arguments and struct members of the wrapper types can be replaced with storing the base types Committee and WorkerCache value.

[mwtian]

Another possible related cleanup is to rename WorkerCache to WorkerConfig, which seems more appropriate.