Bug when performing multiple sync()

[chacha912]

What happened:
When performing multiple 'pushpull' requests, the SDK doesn't work correctly. For example, the following test case fails:

yorkie-js-sdk/test/integration/document_test.ts

Lines 57 to 93 in 394c061

	for (let i = 0; i < 3; i++) {
	it.only('Should work correctly even when "sync" is called multiple times', async function () {
	type TestDoc = { k1: Array<number> };
	const docKey = toDocKey(`${this.test!.title}-${new Date().getTime()}`);
	// 01. c1 attaches d1 and c2 watches same doc.
	const c1 = new yorkie.Client(testRPCAddr);
	await c1.activate();
	const d1 = new yorkie.Document<TestDoc>(docKey);
	d1.update((root) => {
	root['k1'] = [1, 2];
	}, 'set array');
	await c1.attach(d1);
	assert.equal(d1.toSortedJSON(), '{"k1":[1,2]}');
	const c2 = new yorkie.Client(testRPCAddr);
	await c2.activate();
	const d2 = new yorkie.Document<TestDoc>(docKey);
	await c2.attach(d2);
	assert.equal(d2.toSortedJSON(), '{"k1":[1,2]}');
	// 02. c1 updates d1 and removes it.
	d1.update((root) => {
	root['k1'].push(3);
	});
	assert.equal(d1.toSortedJSON(), '{"k1":[1,2,3]}', 'd1');
	await c1.sync();
	// 03. c2 syncs and checks that d2 is removed.
	c2.sync();
	await c2.sync();
	assert.equal(d2.toSortedJSON(), '{"k1":[1,2,3]}', 'd2');
	await c1.deactivate();
	await c2.deactivate();
	});
	}

Anything else we need to know?:

In the test code, four changes are generated:

• change (serverSeq 1): d1 update root['k1'] = [1, 2]
• change (serverSeq 2): c1 initial presence (within c1.attach)
• change (serverSeq 3): c2 initial presence (within c2.attach)
• change (serverSeq 4): d1 update root['k1'].push(3)

When c2.sync() is called, it sends two pushpull requests with checkpoint{clientSeq: 1, serverSeq: 3}. The server responds with change (serverSeq 4), then c2 applies the same change twice, resulting in [1, 2, 3, 3] not [1, 2, 3].
(When multiple changes are pushed, the server handles it to prevent duplicate changes from being stored. However, the client needs to handle it when processing the response.)

I think one of the solution is enqueuing 'sync' requests. The next request should be sent after receiving the response to update the server sequence.

Environment:

• Operating system:
• Browser and version:
• Yorkie version (use yorkie version): 0.4.5
• Yorkie JS SDK version: 0.4.5