[MLS] Hiding content type

[Richard Barnes <[e-mail]>]

Hi all,

Recall that PR#349 does two things (1) use an opaque epoch ID, and (2)
encrypt the content type of the message (application / proposal / commit)
[1].

There was some discussion on the call last week that encrypting the content
type might not be that useful, since an application that relies on a server
to assure ordering of commits will need to at least be able to tell commits
from other things.  Of course, even if the content type is encrypted by
default, the application can add it back in authenticated data, or in some
unauthenticated wrapper.

Net of those considerations, I'm personally still inclined to merge the PR,
to have a conservative baseline.  But those on the call thought it would be
useful to pose this to the group, so please speak up if you have concerns.

--Richard

[1] https://github.com/mlswg/mls-protocol/pull/349