add ExtensionId translator

[lidedongsn]

fix issue #1393 #1399
add ExtensionId translator
Publisher： translate external ExtensionId into internal ExtensionId
Subscriber：translate internal ExtensionId into external ExtensionId

[paul-maxime]

Yes, I had the same problem. The extension IDs are not always the same between each clients, especially between firefox and chrome.
Translating them like the payload IDs is a nice solution.

[lodoyun]

It looks good, I like the idea and the implementation. Just a couple of comments, one does cause issues in my tests.
Thanks!

[zhangshishen]

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0xBE | 0xDE | length=3 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ID | L=0 | data | ID | L=1 | data...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
...data | 0 (pad) | 0 (pad) | ID | L=3 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

I think extension with 1 byte padding will cause error. According to https://tools.ietf.org/html/rfc5285#section-4.2