Rework RPC server start process

[AnnaShaleva]

Is your feature request related to a problem? Please describe.

After #3307 the only node service that requires from caller to send call to Start to a separate goroutine is RPC server. It's so special because it may return errors to the provided error channel, and the caller must start error channel by the moment of RPC server start:

neo-go/cli/server/server.go

Lines 503 to 506 in 327e766

	// Run RPC server in a separate routine. This is necessary to avoid a potential
	// deadlock: Start() can write errors to errChan which is not yet read in the
	// current execution context (see for-loop below).
	go rpcServer.Start()

One day it may cause a bug, because we always need to remember about this RPC server behaviour.

Describe the solution you'd like

Refactor RPC server start so that the caller is able to call Start in the same goroutine. Send code that may return errors to a separate goroutine inside Start.

Describe alternatives you've considered

Allow to return error from Start, but it'd be inconvenient since we have common Service interface and none of our services return error from start.