MINOR: atomic: make sure to always relax after a failed CAS

MINOR: atomic: make sure to always relax after a failed CAS

There were a few places left where we forgot to call __ha_cpu_relax()
after a failed CAS, in the HA_ATOMIC_UPDATE_{MIN,MAX} macros, and in
a few sync_* API macros (the same as above plus HA_ATOMIC_CAS and
HA_ATOMIC_XCHG). Let's add them now.

This could have been a cause of contention, particularly with
process_stream() calling stream_update_time_stats() which uses 8 of them
in a call (4 for the server, 4 for the proxy). This may be a possible
explanation for the high CPU consumption reported in GH issue #2251.

This should be backported at least to 2.6 as it's harmless.