`AsyncHttpTransport` connection reuse

### Problem Statement

Sentry `AsyncHttpTransport` closes the connection after each envelope sent.
This may cause problems on high traffic servers / cloud environments.

Came up in https://github.com/getsentry/sentry-java/issues/6000

### Solution Brainstorm

We could reuse the connection / not close unless there's an error.

Start by locating AsyncHttpTransport and tracing where the connection is closed after an envelope is sent. Review the related Sentry issue 6000 and the transport's existing error handling; done means connections are reused between envelopes and still close when an error requires it.

索引モデルが issue の本文から書いたものです。