Closes #369

• Removes existing minidump queue from BaseUploader so there's only a single queue
• Extends the JSON Store so it can parse Date objects
• Adds PersistedRequestQueue which stores an array of queue item metadata in a Store and request bodies in separate files
  • Since the request body can be large and binary, we avoid serialisation
• Adds an ElectronOfflineNetTransport that extends the ElectronNetTransport and makes it the default transport.
  • When sendRequest fails, it pushes the failed request into the queue.

Tests

Electron network emulation with { offline: true } doesn't appear to work from the main process and invalid URLs take too long to timeout for reliable tests. Instead we have special DSNs which the server responds differently to.

• To test queueing, we return 429
• To test that other status drop events we return 500

Still to do:

• Consider data races with PersistedRequestQueue with overlapping add/pop
• Unit tests for PersistedRequestQueue since it's difficult to test all its features via e2e
• Which error should result in queuing and which should be dropped? Should any errors be left uncaught or only logged?
• Fix logging. I've seen "minidump sent" when it got queued

Other

This PR also introduces caching for Electron binaries because we were getting 503 Egress exceeded errors.