oxenstored: only process domain connections that notify us by events

oxenstored: only process domain connections that notify us by events

Currently, upon receiving an event, oxenstored will always scan/process all
the domain connections (xs rings), disregarding which domain sent that event.
This is rather costy and inefficient. It also shadows and indulges client
for not correctly communicating with us on message/space availability.

With this patch, oxenstore will only scan/process the domain connections
that have correctly notified us by events or have IO actions leftover from
previous communication.

Signed-off-by: Zheng Li <dev@zheng.li>
Reviewed-by: David Scott <dave.scott@citrix.com>