fix: IAM server must start KeepConnectedToMaster for masterClient usage

The IAM server creates and uses a MasterClient but never started
KeepConnectedToMaster, which could cause blocking if IAM config files
have chunks requiring volume lookups.

Problem flow:
  NewIamApiServerWithStore()
    → creates masterClient
    → ❌ NEVER starts KeepConnectedToMaster

  GetS3ApiConfigurationFromFiler()
    → filer.ReadEntry(iama.masterClient, ...)
      → StreamContent(masterClient, ...) if file has chunks
        → masterClient.GetLookupFileIdFunction()
          → GetMaster(ctx) ← BLOCKS indefinitely waiting for connection!

While IAM config files (identity & policies) are typically small and
stored inline without chunks, the code path exists and would block
if the files ever had chunks.

Fix:
  Start KeepConnectedToMaster in background goroutine right after
  creating masterClient, following the documented pattern:

    mc := wdclient.NewMasterClient(...)
    go mc.KeepConnectedToMaster(ctx)

This ensures masterClient is usable if ReadEntry ever needs to
stream chunked content from volume servers.

Note: This bug was dormant because IAM config files are small (<256 bytes)
and SeaweedFS stores small files inline in Entry.Content, not as chunks.
The bug would only manifest if:
  - IAM config grew > 256 bytes (inline threshold)
  - Config was stored as chunks on volume servers
  - ReadEntry called StreamContent
  - GetMaster blocked indefinitely

Now all 9 production MasterClient instances correctly follow the pattern.

weed/iamapi/iamapi_server.go

@@ -56,9 +56,16 @@ func NewIamApiServer(router *mux.Router, option *IamServerOption) (iamApiServer
 }
 
 func NewIamApiServerWithStore(router *mux.Router, option *IamServerOption, explicitStore string) (iamApiServer *IamApiServer, err error) {
+	masterClient := wdclient.NewMasterClient(option.GrpcDialOption, "", "iam", "", "", "", *pb.NewServiceDiscoveryFromMap(option.Masters))
+	
+	// Start KeepConnectedToMaster for volume location lookups
+	// IAM config files are typically small and inline, but if they ever have chunks,
+	// ReadEntry→StreamContent needs masterClient for volume lookups
+	go masterClient.KeepConnectedToMaster(context.Background())
+	
 	configure := &IamS3ApiConfigure{
 		option:       option,
-		masterClient: wdclient.NewMasterClient(option.GrpcDialOption, "", "iam", "", "", "", *pb.NewServiceDiscoveryFromMap(option.Masters)),
+		masterClient: masterClient,
 	}
 
 	s3ApiConfigure = configure