services: update logs flush after services shutdown

Added sync logs for every service separately to provide the ability to
have a custom logger for each service. This commit makes the code follow
the zap usages rules: `Sync calls the underlying Core's Sync method,
flushing any buffered log entries. Applications should take care to
call Sync before exiting.`

Signed-off-by: Ekaterina Pavlova <ekt@morphbits.io>

pkg/network/server.go

@@ -320,6 +320,8 @@ func (s *Server) Shutdown() {
 	}
 	close(s.quit)
 	<-s.relayFin
+
+	_ = s.log.Sync()
 }
 
 // AddService allows to add a service to be started/stopped by Server.