Review and refactor services start and shutdown processes

[AnnaShaleva]

For each node service (consensus, RPC server, notary service, etc.) we need to ensure that:

1. Double-call to Start doesn't hurt service. Usually it can be done via checking isStarted atomic variable.
2. Double-call to Shutdown doesn't hurt the service. Usually it can be done via checking isStarted and isFinished atomic variables.
3. Service is waiting until all its processes are finished before it returns from Shutdown. Usually it can be done via using two channels: shutdownchannel is used to send a signal to the main service loop to start shutdown process and quit channel is used inside the Shutdown method to wait until main service loop is finished and closes quit.

Not all services follow these rules which can cause problems with #1949 and long-term node functionality support.

[AnnaShaleva]

Related to #2566.