OPTIM/MEDIUM: epoll: fuse active events into polled ones during polling changes

OPTIM/MEDIUM: epoll: fuse active events into polled ones during polling changes

When trying to speculatively send data to a server being connected to,
we see the following pattern :

    connect() = EINPROGRESS
    send() = EAGAIN
    epoll_ctl(add, W)
    epoll_wait() = EPOLLOUT
    send() = success
  > epoll_ctl(del, W)
  > recv() = EAGAIN
  > epoll_ctl(add, R)
    recv() = success
    epoll_ctl(del, R)

The reason for the failed recv() call is that the reading was marked
as speculative while we already have a polled I/O there. So we already
know when removing send write poll that the read is pending. Thus,
let's improve this by merging speculative I/O into polled I/O when
polled state changes. The result is now the following as expected :

    connect() = EINPROGRESS
    send() = EAGAIN
    epoll_ctl(add, W)
    epoll_wait() = EPOLLOUT
    send() = success
    epoll_ctl(mod, R)
    recv() = success
    epoll_ctl(del, R)

This is specific to epoll(), it doesn't make much sense at the moment
to do so for other pollers, because the cost of updating them is very
small.

The average performance gain on small requests is of 1.6% in TCP mode,
which is easily explained with the syscall stats below for 10000 forwarded
connections :

Before :
% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
 91.02    0.024608           0     60000         1 epoll_wait
  2.19    0.000593           0     20000           shutdown
  1.52    0.000412           0     10000     10000 connect
  1.36    0.000367           0     29998      9998 sendto
  1.09    0.000294           0     49993           epoll_ctl
  0.93    0.000252           0     50004     20002 recvfrom
  0.79    0.000214           0     20005           close
  0.62    0.000167           0     20001     10001 accept4
  0.25    0.000067           0     20002           setsockopt
  0.13    0.000035           0     10001           socket
  0.10    0.000028           0     10001           fcntl

After:
% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
 87.59    0.024269           0     50012         1 epoll_wait
  3.19    0.000884           0     20000           shutdown
  2.33    0.000646           0     29996      9996 sendto
  2.02    0.000560           0     10005     10003 connect
  1.40    0.000387           0     40013     10013 recvfrom
  1.35    0.000374           0     40000           epoll_ctl
  0.64    0.000178           0     20001     10001 accept4
  0.55    0.000152           0     20005           close
  0.45    0.000124           0     20002           setsockopt
  0.31    0.000086           0     10001           fcntl
  0.17    0.000047           0     10001           socket

Overall :
   -16.6% epoll_wait
   -20%   recvfrom
   -20%   epoll_ctl

On HTTP, the gain is even better :

% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
 80.43    0.015386           0     60006         1 epoll_wait
  4.61    0.000882           0     30000     10000 sendto
  3.74    0.000715           0     20001     10001 accept4
  3.35    0.000640           0     10000     10000 connect
  2.66    0.000508           0     20005           close
  1.34    0.000257           0     30002     10002 recvfrom
  1.27    0.000242           0     30005           epoll_ctl
  1.20    0.000230           0     10000           shutdown
  0.62    0.000119           0     20003           setsockopt
  0.40    0.000077           0     10001           socket
  0.39    0.000074           0     10001           fcntl
willy@wtap:haproxy$ head -15 apres.txt
% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
 83.47    0.020301           0     50008         1 epoll_wait
  4.26    0.001036           0     20005           close
  3.30    0.000803           0     30000     10000 sendto
  2.55    0.000621           0     20001     10001 accept4
  1.76    0.000428           0     10000     10000 connect
  1.20    0.000292           0     10000           shutdown
  1.14    0.000278           0     20001         1 recvfrom
  0.86    0.000210           0     20003           epoll_ctl
  0.71    0.000173           0     20003           setsockopt
  0.49    0.000120           0     10001           socket
  0.25    0.000060           0     10001           fcntl

Overall :
  -16.6% epoll_wait
  -33%   recvfrom
  -33%   epoll_ctl