Send AppendResponse keepalive once per second

[petuhovskiy]

Describe your changes

walproposer sends AppendRequest at least once per second. This patch adds a response to these requests once per second.

Issue ticket number and link

Fixes #4017

Checklist before requesting a review

• I have performed a self-review of my code.
• If it is a core feature, I have added thorough tests.
• Do we need to implement analytics? if so did you add the relevant metrics to the dashboard?
• If this PR requires public announcement, mark it with /release-notes label and add several sentences in this section.

Checklist before merging

• Do not forget to reformat commit message to not include the above checklist

[github-actions]

Test results for 3782fc3:

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debug build: 214 tests run: 204 passed, 0 failed, 10 (full report)

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release build: 214 tests run: 204 passed, 0 failed, 10 (full report)

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[arssher]

Let's drop

                        if self.reply_tx.send(reply).await.is_err() {
                            return Ok(()); // chan closed, streaming terminated
                        }

in case of NoFlushAppendRequest message; we never get/send feedback in this case anyway.

Also this patch doesn't make walreceiver send KAs on its own initiative if nothing happens (which would help detect broken connection), but this is a step forward since as it should fix #4017