server.config should include trust domains

[willscott]

currently, the server.Config struct is not able to fully configure a talek server. The missing piece is that it does not include the information about the trust domains. While a talek server does not need to know the keys of the other servers, it does need to know if it's a leader, and which other servers it should be RPC'ing with. (And eventually, the server keys should probably be used to establish and validate these RPC connections).

This is a self-contained work item to add an []trustDomainConfig to server.Config, update talekutil to fill it, and update the server driver to determine leader and rpc's from it.

[ryscheng]

I think the new consensus is to:

separate centralized.go into 3 logical entities

• frontend servers are only run on the leader trust domain and coordinate replicating writes to all trust domains, and batching PIR requests
• replica servers are run both on leader and follower trust domains. They store shards and perform PIR batch reads.
• coordinator servers track the current global sequence number and calculate the global cuckoo table configuration across shards.

[willscott]

your comment is, i think, a different refactor. This issue was meant to deal with the way the different configuration structs are laid out - while yours is about refactoring the internal structure of the different parts of the server.

[willscott]

This is obsoleted by the frontend/replica refactor in #51