Tree in GroupInfo not optional

[raphaelrobert]

I remember we discussed this in the past, but I don't think we ever spec'ed this out fully:

Right now the whole tree is part of GroupInfo (which in turn is part of Welcome). This can grow quite big in large groups (~17MB with 100k members with the MTI). The practical limitations are:

• The TLS representation of the tree is hard to parse efficiently (no skipping and no a priori information of how many nodes are in the tree)
• Out-of-band transmission (like server assist) is not possible
• Large messages like that are hard to handle on mobile devices, when they have to be downloaded in the background

We should introduce an indirection, like a commit hash of the tree.