team::broadcast_send lacks source completion indication

[Dan Bonachea]

void team::broadcast_send(value, team) currently lacks source completion indication to the broadcast root for the input data. The value is Serializable and taken as an rvalue reference, but the initiating rank still needs a way to discover when it's safe to resume mutating value (especially for reference types with "real" serialization).

The semantics include:

A copy of value will be asynchronously sent to all other ranks in the team.

but this does not guarantee serialization occurs before return from the initiation, and no synchronization object is provided to the root initiator.

[Dan Bonachea]

Resolved by dropping this interface design in 3ac948b.