Remove sync.Mutex from Target's failure tracking hot path:
- failCount int64 → atomic.Int64
- failedUntil int64 → atomic.Int64
- IsAvailable(): Load() instead of Lock/Unlock, eliminating mutex
from every per-target check in every LB Select() call
- RecordFailure(): Add(1) + Store() instead of Lock/Unlock
- RecordSuccess(): Store(0) instead of Lock/Unlock
This removes the only mutex acquisition in the load balancing
selection path. Every Select() call iterates targets and calls
IsAvailable() on each — previously acquiring failMu per target
when MaxFails > 0. Now fully lock-free.
xfy
d03c180f62