Fold a push's follow-up requests together (#393)

A pushed mail change took three round trips: Email/changes beside a
Mailbox/get, then Email/get for what changed, then the list, the open
thread and a second Mailbox/get. Each page of changes now carries its own
Email/get calls by back-reference, and the one Mailbox/get goes out with
it, so a push settles in two. New mail fetched this way is not asked for
again by the notice.

A reply's sessionState that differs from the session is announced once
rather than on every reply, and session refreshes in flight are shared.
The mock's session state now matches the sessionState on its replies, as
Stalwart's does; tying it to the data counter made every reply trigger a
session refresh in development.

server/src/mock/index.ts

@@ -7,6 +7,8 @@ import { createServer, type IncomingMessage, type ServerResponse } from "node:ht
 import { parseOtpauthUrl, verifyTotp } from "../totp.js";
 import { gzipSync } from "node:zlib";
 import { ACCOUNT, MAX_DELAYED_SEND, MOCK_EDITION, MOCK_LOCALE, NO_REGISTRY, Obj, PASS, PERMISSION_SNAPSHOT, PORT, SHARED_ACCOUNT, SHARED_CAPS, USER, account, nextState, state } from "./config.js";
+
+const SESSION_STATE = "1";
 import { PING_FLOOR_SECONDS, addEmail, blobs, calendars, people, principals, putBlob, recount } from "./data.js";
 import { MAX_OBJECTS, MethodError, directory, enforceLimits, resolveRefs } from "./engine.js";
 import { handlers } from "./handlers.js";
@@ -63,7 +65,12 @@ const session = () => ({
   downloadUrl: `http://127.0.0.1:${PORT}/jmap/download/{accountId}/{blobId}/{name}?accept={type}`,
   uploadUrl: `http://127.0.0.1:${PORT}/jmap/upload/{accountId}/`,
   eventSourceUrl: `http://127.0.0.1:${PORT}/jmap/eventsource/?types={types}&closeafter={closeafter}&ping={ping}`,
-  state: String(state.n),
+  /*
+   * The session's own state, which the account's data changes do not move.
+   * It matches the sessionState on every JMAP reply below, as Stalwart's does;
+   * tying it to the data counter made every reply look like a session change.
+   */
+  state: SESSION_STATE,
 });
 
 
@@ -125,7 +132,7 @@ export const server = createServer(async (req, res) => {
     }
     if (touched.size) { nextState(); setTimeout(() => broadcast([...touched, ...(touched.has("Email") ? ["Mailbox", "Thread"] : [])]), 50); }
     res.writeHead(200, { "content-type": "application/json" });
-    return res.end(JSON.stringify({ methodResponses: responses, sessionState: "1" }));
+    return res.end(JSON.stringify({ methodResponses: responses, sessionState: SESSION_STATE }));
   }
   if (url.pathname.startsWith("/jmap/upload/") && req.method === "POST") {
     const data = await readBody(req);