Fix DNS servers with same tag wrongly merged

[Vigilans]

Fixes #2981

[Vigilans]

For certain settings (identified by path), do not use "tag" field to search for tag. "_tag" is preserved.

[povsister]

According to documentation, mergeSameTag should only be applied on inbounds and outbounds.
This looks like a "workaround" instead of a real fix for that.

[Vigilans]

I've tried to apply it to only inbounds and outbounds, but it will actually break this test that also applies to routing config:

v2ray-core/infra/conf/merge/merge_test.go

Lines 60 to 99 in 26ea22a

	func TestMergeTag(t *testing.T) {
	json1 := `
	{
	"routing": {
	"rules": [{
	"tag":"1",
	"inboundTag": ["in-1"],
	"outboundTag": "out-1"
	}]
	}
	}
	`
	json2 := `
	{
	"routing": {
	"rules": [{
	"_tag":"1",
	"inboundTag": ["in-2"],
	"outboundTag": "out-2"
	}]
	}
	}
	`
	expected := `
	{
	"routing": {
	"rules": [{
	"tag":"1",
	"inboundTag": ["in-1", "in-2"],
	"outboundTag": "out-2"
	}]
	}
	}
	`
	m, err := merge.JSONs([][]byte{[]byte(json1), []byte(json2)})
	if err != nil {
	t.Error(err)
	}
	assertResult(t, m, expected)
	}

[xiaokangwang]

I think this merge request is ready to be merged. To be fair I didn't fully understood the implementation details, but the test seems fine.