Epoll file descriptors that are added to a shared wakeup source are always
added in a non-exclusive manner. That means that when we have multiple epoll
fds attached to a shared wakeup source they are all woken up. This can
lead to excessive cpu usage and uneven load distribution.
This patch introduces two new 'events' flags that are intended to be used
with EPOLL_CTL_ADD operations. EPOLLEXCLUSIVE, adds the epoll fd to the event
source in an exclusive manner such that the minimum number of threads are
woken. EPOLLROUNDROBIN, which depends on EPOLLEXCLUSIVE also being set, can
also be added to the 'events' flag, such that we round robin around the set
of waiting threads.
An implementation note is that in the epoll wakeup routine,
'ep_poll_callback()', if EPOLLROUNDROBIN is set, we return 1, for a successful
wakeup, only when there are current waiters. The idea is to use this additional
heuristic in order minimize wakeup latencies.