epoll: edge triggered and multi-threaded epoll

epoll: edge triggered and multi-threaded epoll

- edge triggered (oneshot) polling with epoll
- pick one event to avoid multiple events getting picked up by same
  thread
  and so get better distribution of events against multiple threads
- wire support for multiple poll threads to epoll_wait in parallel
- evdata to store absolute index and not hint for epoll
- store index and gen of slot instead of fd and index hint
- perform fd close asynchronously inside event.c for multithread safety
- poll is still single threaded

Change-Id: I536851dda0ab224c5d5a1b130a571397c9cace8f
BUG: 1104462
Signed-off-by: Anand Avati <avati@redhat.com>
Signed-off-by: Vijaikumar M <vmallika@redhat.com>
Signed-off-by: Jeff Darcy <jdarcy@redhat.com>
Signed-off-by: Shyam <srangana@redhat.com>
Reviewed-on: http://review.gluster.org/3842
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Raghavendra G <rgowdapp@redhat.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>