BUG/MEDIUM: checks: Subscribe to I/O events on an unfinished connect

BUG/MEDIUM: checks: Subscribe to I/O events on an unfinished connect

In tcp-check based health check, when a new connection is opened, we must wait
it is really established before moving to the next rule. But at this stage, we
must also be sure to subscribe to I/O events. Otherwise, depending on the
timing, the health check may remains sleepy till the timeout.

No backport needed. This patch should fix the issue #622.

(cherry picked from commit b2a4c0d473e3c5dcb87f7d16f2ca410bafc62f64)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>