Send cached and pending envelopes when an envelope is going to be sent

Related to https://github.com/getsentry/sentry-java/issues/912#issuecomment-1057079937

iOS already does it.

The use case is, you capture envelope A and you have no connection, the envelope is cached and won't be sent until you restart your app.
If you didn't restart your app but you captured B another envelope, before sending envelope B, it's gonna read from the disk all cached and pending envelopes (read envelope A here) and send them in bulk.