Execute keep alive response

Clarify and document the behavior for using an operation response with done=false. This has not been documented with expected behavior by the server or client, and should be detailed as such.

Client behavior should be to ignore the update and continue to wait for responses.

Server behavior should be to emit this update on some reasonable interval - with or without updates, perhaps upon some (maximally limited) client request period, since this will vary by client and should be tunable to address connectivity issues over multi-hop routes.

Review the operation response definition and existing client and server documentation for the done=false case. Clarify the expected client behavior and establish a bounded, tunable interval for server keep-alive updates, then document the resulting protocol behavior for both sides.

Written by the indexing model from the issue text.