server: Optimize sending for SubscriptionSink::pipe_from_stream

[lexnv]

The SubscriptionSink::send contains the logic for:

• accepting the subscription if not previously accepted
• building and sending the response to the client

While this offers ergonomic benefits for the server users not needing
to handle the accepting of the subscription, this function is exercised
during SubscriptionSink::pipe_from_stream.

The SubscriptionSink::pipe_from_stream first accepts the subscription
if previously not accepted, then it submits to the client side each element
from the provided stream. The sending of elements can be optimized
to not accept the subscription because it was priorly accepted.

This PR creates an inner private helper function for sending the response
to the client without accepting the subscription.