New Groups support

[jagerman]

Adds a group info config type, which tracks distributed info for v2 groups.

This requires introducing/using a few new concepts not currently used for user config messages:

• Multiple decryption keys. User config doesn't do this at all (rather it generates a single decryption key from the private key for each namespace).

This doesn't yet add support for sharing and distributing those keys, just for having being able to load a config with a list of multiple possible keys.

• Config signing and verification. For user configs this isn't done, since only the owner can actually encrypt/decrypt a config message, just being able to decrypt it is authentication enough.

This required various modifications to make the config library properly prevent modifications when we can't modify, and to properly follow that through in terms of merging, updates, etc.

TODO:

• [x] storage server subaccount auth signature generation
• [x] storage server subaccount authentication helper
• [x] C API for user groups group_info
• [x] convo info volatile entry for group
• [x] support for wrapping/unwrapping a config in a protobuf wrapper, for existing config types (moved to PR #51 instead of here)
• [x] add members/keys size() functions for group types

[frtget]

should have done everything in a single review, sorry about that :)

[venezuela01]

We need more experts to review the design and implementation. I hope @KeeJef will publish more internal design drafts before an actual software release. We have seen examples in the past where in-house developed code was vulnerable to algebraic attacks due to subtle mistakes by the core team. This situation further evidences the need for more external volunteers like @frtget to provide valuable opinions.