services: use buffered channels for block subscription

Add a tiny buffer where possible to avoid Blockchain's blocking
on new block addition.

pkg/consensus/consensus.go

@@ -89,7 +89,8 @@ type service struct {
 	messages     chan Payload
 	transactions chan *transaction.Transaction
 	// blockEvents is used to pass a new block event to the consensus
-	// process.
+	// process. It has a tiny buffer in order to avoid Blockchain blocking
+	// on block addition under the high load.
 	blockEvents  chan *coreb.Block
 	lastProposal []util.Uint256
 	wallet       *wallet.Wallet

pkg/services/notary/notary.go

@@ -65,6 +65,11 @@ type (
 		mp *mempool.Pool
 		// requests channel
 		reqCh chan mempoolevent.Event
+		// blocksCh is a channel used to receive block notifications from the
+		// Blockchain. It is not buffered intentionally, as it's important to keep
+		// the notary request pool in sync with the current blockchain heigh, thus,
+		// it's not recommended to use a large size of notary requests pool as it may
+		// slow down the block processing.
 		blocksCh chan *block.Block
 		stopCh   chan struct{}
 		done     chan struct{}

pkg/services/stateroot/service.go

@@ -63,6 +63,9 @@ type (
 		timePerBlock    time.Duration
 		maxRetries      int
 		relayExtensible RelayCallback
+		// blockCh is a channel used to receive block notifications from the
+		// Blockchain. It has a tiny buffer in order to avoid Blockchain blocking
+		// on block addition under the high load.
 		blockCh chan *block.Block
 		stopCh  chan struct{}
 		done    chan struct{}
@@ -84,7 +87,7 @@ func New(cfg config.StateRoot, sm *stateroot.Module, log *zap.Logger, bc Ledger,
 		chain:           bc,
 		log:             log,
 		incompleteRoots: make(map[uint32]*incompleteRoot),
-		blockCh:         make(chan *block.Block),
+		blockCh:         make(chan *block.Block, 1),
 		stopCh:          make(chan struct{}),
 		done:            make(chan struct{}),
 		timePerBlock:    bcConf.TimePerBlock,