Add support for multi-reply handlers

[real-alexei]

@sidorares Now it's possible to keep handlers as needed. For example, to track input events with a Record extension (e.g. EnableContext):

ext.EnableContext = function(ctx, cb) {
  X.seq_num++;
  X.pack_stream.pack('CCSL', [ext.majorOpcode, 5, 2, ctx]);
  X.replies[X.seq_num] = [
    handler,
    cb,
    true // multiReply
  ];
  X.pack_stream.flush();
}

[sidorares]

so the callback decides when last packet arrives? Looks wrong to me, should be in the handler code

[real-alexei]

Introduced unsubscribe callback to work with core request templates.

[real-alexei]

Should I rename the callback to removeHandler?

[sidorares]

Hey @aselivanov sorry this was silent for months.
Would like to merge this but really need to add some tests

@santigimeno maybe you could assist?

[santigimeno]

Fix indentation

[santigimeno]

@sidorares sure
@aselivanov yes, some tests are needed. Let me know if you need help with them