rpc/server: add notification subscription

Note that the protocol differs a bit from #895 in its notifications format,
to avoid additional server-side processing we're omitting some metadata like:
 * block size and confirmations
 * transaction fees, confirmations, block hash and timestamp
 * application execution doesn't have ScriptHash populated

Some block fields may also differ in encoding compared to `getblock` results
(like nonce field).

I think these differences are unnoticieable for most use cases, so we can
leave them as is, but it can be changed in the future.

pkg/rpc/server/subscription.go

@@ -0,0 +1,35 @@
+package server
+
+import (
+	"github.com/gorilla/websocket"
+	"github.com/nspcc-dev/neo-go/pkg/rpc/response"
+)
+
+type (
+	// subscriber is an event subscriber.
+	subscriber struct {
+		writer chan<- *websocket.PreparedMessage
+		ws     *websocket.Conn
+
+		// These work like slots as there is not a lot of them (it's
+		// cheaper doing it this way rather than creating a map),
+		// pointing to EventID is an obvious overkill at the moment, but
+		// that's not for long.
+		feeds [maxFeeds]response.EventID
+	}
+)
+
+const (
+	// Maximum number of subscriptions per one client.
+	maxFeeds = 16
+
+	// This sets notification messages buffer depth, it may seem to be quite
+	// big, but there is a big gap in speed between internal event processing
+	// and networking communication that is combined with spiky nature of our
+	// event generation process, which leads to lots of events generated in
+	// short time and they will put some pressure to this buffer (consider
+	// ~500 invocation txs in one block with some notifications). At the same
+	// time this channel is about sending pointers, so it's doesn't cost
+	// a lot in terms of memory used.
+	notificationBufSize = 1024
+)