Make server.py and distributed.py Server class naming more clear.

[coledie]

Currently we have distributed.py::Server that wraps server.py::Server which is not clear naming. Should also make docs clear, the purpose of distributed.py is to have a client that works with frameworks that serialize/deserialize on new epoch and a server that reads meta partition count and makes it easier to start grpc servers so it can determine number of shards. e.g. sometimes users know total number of partitions and then distributed is not needed.