BUG/MEDIUM: peers: reset starting point if peers appears longly disconnected

BUG/MEDIUM: peers: reset starting point if peers appears longly disconnected

If two peers are disconnected and during this period they continue to
process a large amount of local updates, after a reconnection they
may take a long time before restarting to push their updates. because
the last pushed update would appear internally in futur.

This patch fix this resetting the cursor on acked updates at the maximum
point considered in the past if it appears in futur but it means we
may lost some updates. A clean fix would be to update the protocol to
be able to signal a remote peer that is was not updated for a too long
period and needs a full resync but this is not yet supported by the
protocol.

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

(cherry picked from commit d9729da98262f2136ad4eac44c3ec2f710cb4a49)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>