Fix race conditions and make Close() idempotent

Address CodeRabbit review #3512078995:

1. **Critical: Fix unsynchronized read in error message**
   - Line 584 read len(fc.filerAddresses) without lock
   - Race with refreshFilerList appending to slice
   - Fixed: Take RLock to read length safely
   - Prevents race detector warnings

2. **Important: Make Close() idempotent**
   - Closing already-closed channel panics
   - Can happen with layered cleanup in shutdown paths
   - Fixed: Use sync.Once to ensure single close
   - Safe to call Close() multiple times now

3. **Nitpick: Add warning for empty filer address**
   - getFilerAddress() can return empty string
   - Helps diagnose unexpected state
   - Added: Warning log when no filers available

4. **Nitpick: Guard deprecated index-based helpers**
   - shouldSkipUnhealthyFiler, recordFilerSuccess/Failure
   - Accessed filerHealth without lock (races with discovery)
   - Fixed: Take RLock and check bounds before array access
   - Prevents index out of bounds and races

Changes:
- filer_client.go:
  - Add closeDiscoveryOnce sync.Once field
  - Use Do() in Close() for idempotent channel close
  - Add RLock guards to deprecated index-based helpers
  - Add bounds checking to prevent panics
  - Synchronized read of filerAddresses length in error

- s3api_server.go:
  - Add warning log when getFilerAddress returns empty

Benefits:
- No race conditions (passes race detector)
- No panic on double-close
- Better error diagnostics
- Safe with discovery enabled
- Production-hardened shutdown logic

weed/s3api/s3api_server.go

@@ -221,6 +221,7 @@ func (s3a *S3ApiServer) getFilerAddress() pb.ServerAddress {
 	if len(s3a.option.Filers) > 0 {
 		return s3a.option.Filers[0]
 	}
+	glog.Warningf("getFilerAddress: no filer addresses available")
 	return ""
 }