server: factor and simplify the network setup logic

This patch performs the following cleanups and refactors:

- adds explanatory comments;
- factors the call to net.Listen and starting the server
  for the admin UI into a new method `(*Server).startServeUI()`.
- factors the call to net.Listen and starting the RPC server
  into a new method `(*Server).startServeRPC()`.
- factors the start of the SQL server into a new method
  `(*Server).startServeSQL()`.
- extracts the TCP keepalive configuration function
  into a top-level function (was a closure).

Release note: None