## 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`.
If WAL truncation fails in the middle it might leave some data on disk
above the write/flush LSN. In theory, concatenated with previous records
it might form bogus WAL (though very unlikely in practice because CRC
would protect from that). To protect from that, set
pending_wal_truncation flag: means before any WAL writes truncation must
be retried until it succeeds. We already did that in case of safekeeper
restart, now extend this mechanism for failures without restart. Also,
importantly, reset LSNs in the beginning of the operation, not in the
end, because once on disk deletion starts previous pointers are wrong.
All this most likely haven't created any problems in practice because
CRC protects from the consequences.
Tests for this are hard; simulation infrastructure might be useful here
in the future, but not yet.
## Problem
`TimelinePersistentState::empty()`, used for tests and benchmarks, had a
hardcoded 16 MB WAL segment size. This caused confusion when attempting
to change the global segment size.
## Summary of changes
Inherit from `WAL_SEGMENT_SIZE` in `TimelinePersistentState::empty()`.
## Problem
The control file is flushed on the WAL ingest path when the commit LSN
advances by one segment, to bound the amount of recovery work in case of
a crash. This involves 3 additional fsyncs, which can have a significant
impact on WAL ingest throughput. This is to some extent mitigated by
`AppendResponse` not being emitted on segment bound flushes, since this
will prevent commit LSN advancement, which will be addressed separately.
## Summary of changes
Don't flush the control file on the WAL ingest path at all. Instead,
leave that responsibility to the timeline manager, but ask it to flush
eagerly if the control file lags the in-memory commit LSN by more than
one segment. This should not cause more than `REFRESH_INTERVAL` (300 ms)
additional latency before flushing the control file, which is
negligible.
## Problem
We don't have a metric capturing the latency of segment initialization.
This can be significant due to fsyncs.
## Summary of changes
Add an `initialize_segment` variant of
`safekeeper_wal_storage_operation_seconds`.
