Set timestamp of events before sending

`timestamp` is documented as one of the required event attributes along with `event_id` and others. See https://develop.sentry.dev/sdk/event-payloads/overview/.

Other SDKs set it in the client, e.g. Python:

https://github.com/getsentry/sentry-python/blob/@{2020.05.06}/sentry_sdk/client.py#L143-L152