network: wait for the relayer thread to finish on shutdown

Unsubscribe and drain first, then return from the Shutdown method. It's
important wrt to subsequent chain shutdown process (normally it's closed right
after the network server).

pkg/network/server.go

@@ -121,6 +121,7 @@ type (
 		register   chan Peer
 		unregister chan peerDrop
 		quit       chan struct{}
+		relayFin   chan struct{}
 
 		transactions chan *transaction.Transaction
 
@@ -170,6 +171,7 @@ func newServerFromConstructors(config ServerConfig, chain Ledger, stSync StateSy
 		id:             randomID(),
 		config:         chain.GetConfig(),
 		quit:           make(chan struct{}),
+		relayFin:       make(chan struct{}),
 		register:       make(chan Peer),
 		unregister:     make(chan peerDrop),
 		txInMap:        make(map[util.Uint256]struct{}),
@@ -273,6 +275,7 @@ func (s *Server) Shutdown() {
 		s.notaryRequestPool.StopSubscriptions()
 	}
 	close(s.quit)
+	<-s.relayFin
 }
 
 // AddService allows to add a service to be started/stopped by Server.
@@ -1433,6 +1436,7 @@ drainBlocksLoop:
 		}
 	}
 	close(ch)
+	close(s.relayFin)
 }
 
 // verifyAndPoolTX verifies the TX and adds it to the local mempool.