## Problem
When processing pipelined `AppendRequest`s, we explicitly flush the WAL
every second and return an `AppendResponse`. However, the WAL is also
implicitly flushed on segment bounds, but this does not result in an
`AppendResponse`. Because of this, concurrent transactions may take up
to 1 second to commit and writes may take up to 1 second before sending
to the pageserver.
## Summary of changes
Advance `flush_lsn` when a WAL segment is closed and flushed, and emit
an `AppendResponse`. To accommodate this, track the `flush_lsn` in
addition to the `flush_record_lsn`.
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