BUG/MINOR: sink: retry attempt for sft server may never occur

BUG/MINOR: sink: retry attempt for sft server may never occur

Since 9561b9fb6 ("BUG/MINOR: sink: add tempo between 2 connection
attempts for sft servers"), there is a possibility that the tempo we use
to schedule the task expiry may point to TICK_ETERNITY as we add ticks to
tempo with a simple addition that doesn't take care of potential wrapping.

When this happens (although relatively rare, since now_ms only wraps every
49.7 days, but a forced wrap occurs 20 seconds after haproxy is started
so it is more likely to happen there), the process_sink_forward() task
expiry being set to TICK_ETERNITY, it may never be called again, this
is especially true if the ring section only contains a single server.

To fix the issue, we must use tick_add() helper function to set the tempo
value and this way we ensure that the value will never be TICK_ETERNITY.

It must be backported everywhere 9561b9fb6 was backported (up to 2.6
it seems).