MINOR: counters: Introduce COUNTERS_UPDATE_MAX()

MINOR: counters: Introduce COUNTERS_UPDATE_MAX()

Introduce COUNTERS_UPDATE_MAX(), and use it instead of using
HA_ATOMIC_UPDATE_MAX() directly.
For now it just calls HA_ATOMIC_UPDATE_MAX(), but will later be modified
so that we can disable max calculation.
This can be backported up to 2.8 if the usage of COUNTERS_UPDATE_MAX()
generates too many conflicts.