Improve Envelope Caching process

### Description

As of now [we save envelopes to disk right before try to send them](https://github.com/getsentry/sentry-java/blob/0bd723b1c59af639c4a86bbade526e0108096f8e/sentry/src/main/java/io/sentry/transport/AsyncHttpTransport.java#L227-L247).
In case of a slow network connection this could lead to envelopes queuing up and being potentially lost in case of an app termination.

We should try to store to disk as early as possible, and then enqueue the envelope for sending over the wire.