simplify: Rely on in-memory commit as source of truth for offsets

INSIGHT:
User correctly pointed out: 'kafka gateway should just use the SMQ async
offset committing' - we shouldn't manually create goroutines to wrap SMQ.

REVISED APPROACH:
1. **In-memory commit** is the primary source of truth
   - Immediate response to client
   - Consumers rely on this for offset tracking
   - Fast < 1ms operation

2. **SMQ persistence** is best-effort for durability
   - Used for crash recovery when in-memory lost
   - Sync call (no manual goroutine wrapping)
   - If it fails, not fatal - in-memory is current state

DESIGN:
- In-memory: Authoritative, always succeeds (or client sees error)
- SMQ storage: Durable, failure is logged but non-fatal
- Auto-commit: Periodically pushes offsets to SMQ
- Manual commit: Explicit confirmation of offset progress

This matches Kafka semantics where:
- Broker always knows current offsets in-memory
- Persistent storage is for recovery scenarios
- No artificial blocking on persistence

EXPECTED BEHAVIOR:
- Fast offset response (unblocked by SMQ writes)
- Durable offset storage (via SMQ periodic persistence)
- Correct offset recovery on restarts
- No message loss or duplicates when offsets committed

weed/mq/kafka/protocol/offset_management.go

@@ -149,7 +149,7 @@ func (h *Handler) handleOffsetCommit(correlationID uint32, apiVersion uint16, re
 
 		for _, p := range t.Partitions {
 
-			// Create consumer offset key for SMQ storage
+			// Create consumer offset key for SMQ storage (not used immediately)
 			key := ConsumerOffsetKey{
 				Topic:                 t.Name,
 				Partition:             p.Index,
@@ -157,29 +157,26 @@ func (h *Handler) handleOffsetCommit(correlationID uint32, apiVersion uint16, re
 				ConsumerGroupInstance: req.GroupInstanceID,
 			}
 
-			// Commit offset to in-memory first for immediate client response
-			// Then commit to persistent storage asynchronously to avoid blocking
+			// Commit offset synchronously for immediate consistency
 			var errCode int16 = ErrorCodeNone
 			if generationMatches {
-				// Store in in-memory map first (fast path - enables immediate client response)
+				// Store in in-memory map for immediate response
+				// This is the primary committed offset position for consumers
 				if err := h.commitOffset(group, t.Name, p.Index, p.Offset, p.Metadata); err != nil {
 					errCode = ErrorCodeOffsetMetadataTooLarge
-					glog.V(2).Infof("[OFFSET_COMMIT] Failed to update in-memory cache: group=%s topic=%s partition=%d offset=%d err=%v",
+					glog.V(2).Infof("[OFFSET_COMMIT] Failed to commit offset: group=%s topic=%s partition=%d offset=%d err=%v",
 						req.GroupID, t.Name, p.Index, p.Offset, err)
 				} else {
-					// Also store in SMQ persistent storage asynchronously (non-blocking)
-					// This ensures recovery after restarts while not blocking the consumer
-					go func(k ConsumerOffsetKey, off int64, meta string) {
-						if err := h.commitOffsetToSMQ(k, off, meta); err != nil {
-							glog.V(3).Infof("[OFFSET_COMMIT] Background persist failed: group=%s topic=%s partition=%d offset=%d err=%v",
-								k.ConsumerGroup, k.Topic, k.Partition, off, err)
-						} else {
-							glog.V(4).Infof("[OFFSET_COMMIT] Background persist succeeded: group=%s topic=%s partition=%d offset=%d",
-								k.ConsumerGroup, k.Topic, k.Partition, off)
-						}
-					}(key, p.Offset, p.Metadata)
-					
-					glog.V(3).Infof("[OFFSET_COMMIT] Committed in-memory: group=%s topic=%s partition=%d offset=%d gen=%d (persistent async)",
+					// Also persist to SMQ storage for durability across broker restarts
+					// This is done synchronously to ensure offset is not lost
+					if err := h.commitOffsetToSMQ(key, p.Offset, p.Metadata); err != nil {
+						// Log the error but don't fail the commit
+						// In-memory commit is the source of truth for active consumers
+						// SMQ persistence is best-effort for crash recovery
+						glog.V(3).Infof("[OFFSET_COMMIT] SMQ persist failed (non-fatal): group=%s topic=%s partition=%d offset=%d err=%v",
+							req.GroupID, t.Name, p.Index, p.Offset, err)
+					}
+					glog.V(3).Infof("[OFFSET_COMMIT] Committed: group=%s topic=%s partition=%d offset=%d gen=%d",
 						req.GroupID, t.Name, p.Index, p.Offset, group.Generation)
 				}
 			} else {