flux_event_[un]subscribe should return a future

[garlick]

flux_event_subscribe() and flux_event_unsubscribe() hide synchronous RPCs in some cases, therefore it should be changed to return a future so that it can be used in reactive programs without blocking the reactor loop.

[garlick]

Because zeromq SUB sockets have a non-blocking subscribe/unsubscribe operation, flux_event_[un]subscribe() is wired directly to the connector operations:

struct flux_handle_ops {
    ...
    int         (*event_subscribe)(void *impl, const char *topic);
    int         (*event_unsubscribe)(void *impl, const char *topic);
}

Yet the shmem connector, currently the only connector based on zeromq sockets, is based on a PAIR socket not PUB/SUB.

These operations will need to be dropped and the API functions implemented directly in terms of the RPCs formerly hidden within the operations.

[chu11]

As a round 1 refactoring/implementation, discussed having this function return a fulfilled future.

[stale]

This issue has been automatically marked as stale because it has not had activity for 365 days. It will be closed if no further activity occurs within 14 days. Thank you for your contributions.

[garlick]

We have these now so closing.

flux_future_t *flux_event_subscribe_ex (flux_t *h,
                                        const char *topic,
                                        int flags);
flux_future_t *flux_event_unsubscribe_ex (flux_t *h,
                                          const char *topic,
                                          int flags);