changing durability to relaxed

[tropicadri]

After reading #2854 and this article, seeing it is the default behavior in FF that seems to improve the performance, that is supported in Chrome and seems to be soon supported in Safari, I think it is a good idea to change the durability.

Fixes #2854

[jeffposnick]

We also write to IndexedDB (via add() instead of put()) at:

workbox/packages/workbox-background-sync/src/lib/QueueDb.ts

Lines 57 to 60 in 63ea2ca

	async addEntry(entry: UnidentifiedQueueStoreEntry): Promise<void> {
	const db = await this.getDb();
	await db.add(REQUEST_OBJECT_STORE_NAME, entry as QueueStoreEntry);
	}

Should that be moved inside a transaction with relaxed durability as well?