[keyserver] implement socket and update code to interact with Tunnelbroker API

Summary:

1. Updates old code to new API
2. Implement Socket
3. Remove old types

There is some code duplication with what's in lib/. I'll try to refactor this later.

Looks like RefreshKeyRequest is defined not properly in Rust codebase, should not be a separate message but part of MessageToDevice. I'll fix that as followup - shouldn't affect this code.

Depends on D9596

Test Plan:

1. Check if the keyserver can connect to Tunnelbroker
2. Check if the socket is properly closed when eg. Tunnelbroker is killed
3. Check if socket issues don't cause the keyserver to crash
4. Send RefreshKeyRequest and verify on keyserver and Identity logs that it is properly processed
5. Kill keyserver, send RefreshKeyRequest, launch keyserver and check if the message was delivered and processed.

Reviewers: michal, bartek, varun

Reviewed By: michal

Subscribers: ashoat, tomek, wyilio

Differential Revision: https://phab.comm.dev/D9623