dnsdist: Fix a race when accessing a backend health status

dnsdist: Fix a race when accessing a backend health status

While there should not be much risk in a data race involving a boolean
apart from getting an outdated value, it's still undefined behaviour
and it rightfully makes TSAN unhappy.
This commit makes the status atomic: hopefully using relaxed memory
ordering when reading the status will make it as cheap as a regular
non-atomic read on most platforms.