FeedSlaveThread uses blocking connection in libevent improperly

### Search before asking

- [x] I had searched in the [issues](https://github.com/apache/kvrocks/issues) and found no similar issues.

### Motivation

Let's created a separate issue to track: https://github.com/apache/kvrocks/issues/3097#issuecomment-3153004778

In theory, we should only use non-blocking socket in bufferevent. However, we are now essentially using a blocking socket in the read callback for `FeedSlaveThread`

### Solution

_No response_

### Are you willing to submit a PR?

- [x] I'm willing to submit a PR!

Start by locating FeedSlaveThread and its libevent bufferevent read callback, then review the discussion in issue #3097. Trace how the socket is configured and used during the callback; done means the FeedSlaveThread path no longer performs blocking socket operations through bufferevent.

Written by the indexing model from the issue text.