Rename classes in shuffle RPC used for block push operations

In the current implementation of push-based shuffle, we are reusing certain code between both block fetch and block push.

This is generally good except that certain classes that are meant to be used for both block fetch and block push now have names that indicate they are only for block fetches, which is confusing.

This ticket renames these classes to be more generic to be reused across both block fetch and block push.