fix: restore tracing endpoint validation; honor per-side client timeouts; cache eviction target; redis close

- tracing: validateEndpoint used fmt.Sscanf("%s:%d") which treats %s as
  greedy, so every valid host:port endpoint failed and traced==nil disabled
  tracing entirely. Validate with net.SplitHostPort (pure helper, port range
  0-65535); test TestValidateEndpoint.
- proxy: createFasthttpClient used clientTimeout for every timeout slot,
  ignoring the parsed CLIENT_READ_TIMEOUT/CLIENT_WRITE_TIMEOUT (silent
  config loss, documented separately in README). Honour per-side read/write
  timeouts, falling back to client timeout when unset.
- cache/memory: evictToFreeMemory capped candidates at maxCacheSize/5 and
  stopped at the subset end, so a large write could leave the cache over
  its memory limit. Collect all entries and sort.Slice (correct + O(n log n)
  vs O(cap^2)), evict until the target is met or the cache is drained.
- cache/redis: the Redis cache client was never closed at shutdown (unlike
  the metrics aggregator). Add RedisConfig.Close, CacheWrapper.Close, a
  cache.Shutdown and register it with the ShutdownManager.

cache/redis/wrapper.go

@@ -102,3 +102,12 @@ func (w *CacheWrapper) GetMemoryUsage() int64 {
 func (w *CacheWrapper) GetMaxMemorySize() int64 {
 	return 0
 }
+
+// Close releases the underlying Redis client's connection pool at shutdown.
+// Makes CacheWrapper satisfy io.Closer so the cache layer can close it.
+func (w *CacheWrapper) Close() error {
+	if w == nil || w.redis == nil {
+		return nil
+	}
+	return w.redis.Close()
+}