Add assertion for history. Rebase before landing

Make errors more detailed

General logic looks okay.
I initially had mixed feelings about code structure of TunnelbrokerSocket but I have no better ideas. This is something we can always refactor later.

On web, retrieve username consistently everywhere. Rebase before landing

Add console.warn

When rebasing, you'll have to add initialDeviceListparam - see D12017 and the following diffs in that stack (D12067 is relevant here)

When rebasing, you'll have to add initial_device_list to Reserved*RegistrationRequest - see D12017 and the following diffs in that stack

Rebase before landing

In D12017#343209, @ashoat wrote:

Interesting that this change is backwards compatible! What's the significance of the optional keyword in the .proto definition – is that just an annotation so that types can be codegenned correctly?

Looks good now