grpc: don't tear down the shared master connection on a caller's own timeout (#9775)

A Canceled/DeadlineExceeded from the caller's per-request context was
treated like a dead channel: it closed the shared cached ClientConn and
cancelled every other in-flight RPC on it with "the client connection is
closing". Under a burst of concurrent chunk assigns (e.g. a large S3
multipart upload) one slow assign hitting its 10s attempt timeout could
poison the connection for all the rest, cascading into a flood of 500s.

Thread the caller's context into shouldInvalidateConnection and only
invalidate on Canceled/DeadlineExceeded while that context is still live,
which isolates the genuine stale-channel signal (a peer restart behind a
k8s Service VIP). To carry the context, add a ctx parameter to the
existing WithGrpcClient, WithMasterClient, and WithMasterServerClient; the
master assign and volume-lookup paths pass their per-attempt context and
every other caller passes context.Background().

weed/server/master_grpc_server_admin.go

@@ -222,7 +222,7 @@ func (ms *MasterServer) Ping(ctx context.Context, req *master_pb.PingRequest) (r
 		})
 	}
 	if req.TargetType == cluster.MasterType {
-		pingErr = pb.WithMasterClient(false, pb.ServerAddress(req.Target), ms.grpcDialOption, false, func(client master_pb.SeaweedClient) error {
+		pingErr = pb.WithMasterClient(context.Background(), false, pb.ServerAddress(req.Target), ms.grpcDialOption, false, func(client master_pb.SeaweedClient) error {
 			pingResp, err := client.Ping(ctx, &master_pb.PingRequest{})
 			if pingResp != nil {
 				resp.RemoteTimeNs = pingResp.StartTimeNs