Early EndDataStream RPC could be responded earlier

When a producer fragment sends no rows and finishes before the receiver is initialized the EndDataStream rpc is stored as early sender and is responded when the receiver is registered.

https://github.com/apache/impala/blob/effc9df933b46eb5b0acf55a858606415425505f/be/src/runtime/krpc-data-stream-mgr.cc#L150

While it is important to store the information that the EOS has happened to unregister the sender from the receiver, the RPC itself could be responded right after it was stored in the early sender map.