receiver_proxy represents a receiver
that will be notified
by the implementations of parallel_scheduler_backend
to trigger the completion operations.
bulk_item_receiver_proxy is derived from receiver_proxy and
is used for bulk_chunked and bulk_unchunked customizations
that will also receive notifications
from implementations of parallel_scheduler_backend
corresponding to different iterations.
Remarks: get_stop_token_t is
in the implementation-defined set of supported queries, and
inplace_stop_token is a member
of the implementation-defined set of supported result types
for get_stop_token_t.