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/replication/source/filer_source.go

@@ -149,7 +149,7 @@ var _ = filer_pb.FilerClient(&FilerSource{})
 
 func (fs *FilerSource) WithFilerClient(streamingMode bool, fn func(filer_pb.SeaweedFilerClient) error) error {
 
-	return pb.WithGrpcClient(streamingMode, fs.signature, func(grpcConnection *grpc.ClientConn) error {
+	return pb.WithGrpcClient(context.Background(), streamingMode, fs.signature, func(grpcConnection *grpc.ClientConn) error {
 		client := filer_pb.NewSeaweedFilerClient(grpcConnection)
 		return fn(client)
 	}, fs.grpcAddress, false, fs.grpcDialOption)