perf: JSON-RPC: faster eventLoop: request buffer

Before this commit we would not start another `getNextEvent()`
before we have received a response for the previous one.
This commit makes a pool of `getNextEvent()` requests
so that the server always has a request
that it can readily respond to.

Measurements

I have tested this on Delta Chat desktop.
This seems to provide a measurable speedup in handling event bursts.

I did `console.time()` when we start
a `BackendRemote.rpc.maybeNetwork()` request
(which we do every time the main window is focused),
and a `console.timeEnd()` when we receive the first
"IDLE entering wait-on-remote state" entry, inside of
[`ipcBackend.on('json-rpc-message'`](https://github.com/deltachat/deltachat-desktop/blob/3846aef67c1644d8ec8b57b229329855a378ee4e/packages/target-electron/runtime-electron/runtime.ts#L52).
For these measurements I also disabled request-response logging
with `config['log-debug'] = false`.

Each such `maybeNetwork()` resulted in ~1000 `getNextEvent()` responses.

With the original event loop (without a pool) the average time
based on 150 measurements was 1152.28 ms.
With the new event loop with a pool of 20
based on 150 measurements it was 774.58 ms.

That is 67.22% of the original average duration.

Related:
- deltachat/deltachat-desktop#5282

Author: WofWca

deltachat-jsonrpc/typescript/src/client.ts

@@ -40,20 +40,51 @@ export class BaseDeltaChat<
      * and emitting the respective events on this class.
      */
     startEventLoop: boolean,
+    options?: {
+      /**
+       * @see {@linkcode BaseDeltaChat.eventLoop}.
+       *
+       * Has no effect if {@linkcode startEventLoop} === false.
+       */
+      eventLoopRequestPoolSize?: number;
+    },
   ) {
     super();
     this.rpc = new RawClient(this.transport);
     if (startEventLoop) {
-      this.eventTask = this.eventLoop();
+      this.eventTask = this.eventLoop({
+        eventLoopRequestPoolSize: options?.eventLoopRequestPoolSize,
+      });
     }
   }
 
   /**
    * @see the constructor's `startEventLoop`
    */
-  async eventLoop(): Promise<void> {
+  async eventLoop(options?: {
+    /**
+     * How many {@linkcode RawClient.getNextEvent} to constantly keep open.
+     * Having a value > 1 improves performance
+     * when dealing with bursts of events.
+     *
+     * Must be >= 1.
+     *
+     * @default 20
+     */
+    eventLoopRequestPoolSize?: number;
+  }): Promise<void> {
+    const promises: ReturnType<typeof this.rpc.getNextEvent>[] = [];
+    for (let i = 0; i < (options?.eventLoopRequestPoolSize ?? 20); i++) {
+      promises.push(this.rpc.getNextEvent());
+    }
+    const bufferLength = promises.length;
+    let currInd = 0;
+
     while (true) {
-      const event = await this.rpc.getNextEvent();
+      const event = await promises[currInd];
+      promises[currInd] = this.rpc.getNextEvent();
+      currInd = (currInd + 1) % bufferLength;
+
       //@ts-ignore
       this.emit(event.event.kind, event.contextId, event.event);
       this.emit("ALL", event.contextId, event.event);