BUG/MINOR: checks: Handle synchronous connect when a tcpcheck is started

BUG/MINOR: checks: Handle synchronous connect when a tcpcheck is started

A connection may be synchronously established. In the tcpcheck context, it
may be a problem if several connections come one after another. In this
case, there is no event to close the very first connection before starting
the next one. The checks is thus blocked and timed out, a L7 timeout error
is reported.

To fix the bug, when a tcpcheck is started, we immediately evaluate its
state. Most of time, nothing is performed and we must wait. But it is thus
possible to handle the result of a successfull connection.

This patch should fix the issue #1234. It must be backported as far as 2.2.