[vm-service] Make sure socket IDs are unique in SocketProfile

[alexmarkov]

SocketStatistic, which is a part of SocketProfile returned by getSocketProfile vm-service API describes socket id in the following way (https://github.com/dart-lang/sdk/blob/main/runtime/vm/service/service_extension.md#socketstatistic):

  // The unique ID associated with this socket.
  int id;

However, the implementation uses address of Socket C++ objects as those ids. Those objects can be freed and new Socket objects could be allocated at the same addresses, making those ids non-unique.

(This actually happens with new async/await implementation on service_2/network_profiling_test as async functions no longer capture everything in the context and socket objects can be garbage-collected earlier.)

@bkonyi

[alexmarkov]

Note that network_profiling_test.dart now has a workaround for this bug:

sdk/runtime/observatory/tests/service/network_profiling_test.dart

Lines 59 to 63 in e11d6b4

	// Workaround for dartbug.com/49111: make sure socket IDs are not reused.
	sockets.add(serverSocket);
	sockets.add(socket);
	sockets.add(server);
	sockets.add(client);

It can be removed when this issue is fixed.