BUG/MEDIUM: peers: stop considering ack messages teaching a full resync

BUG/MEDIUM: peers: stop considering ack messages teaching a full resync

The re-con cursor was updated receiving any ack message
even if we are pushing a complete resync to a peer. This cursor
is reset at the end of the resync but if the connection is broken
during resync, we could re-start at an unwanted point.

With this patch, the peer stops to consider ack messages pushing
a resync since the resync process has is own acknowlegement and
is always restarted from the beginning in case of broken connection.

This patch should be backported on all supported branches ( >= 1.6 )

(cherry picked from commit b0d60bed36cfbfead5a35e6a9520e8d5e9345a7f)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>