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.

沒有指定檔案或測試。先從 envelope 的傳送路徑，以及磁碟快取和待處理 envelope 的處理著手，接著與 issue 中提到的 iOS 行為進行比較。完成的標準是：一次傳送嘗試也會載入已快取和待處理的 envelope，並批次傳送它們，而不需要重新啟動應用程式。

由索引模型根據 Issue 內容生成。