release-20.1: kv/kvserver: Remove buffering from SSTSnapshotStorageFile

[itsbilal]

Backport 1/1 commits from #50831.

/cc @cockroachdb/release

---

When investigating a durability violation around ingested sstables
that disappeared after a power outage, we realized that
SSTSnapshotStorageFile does not flush its buffer before doing
a Sync(). When used with a pebble SSTWriter, we expect Sync()
to ensure all bytes written to that file are synced to disk,
which is not what was happening here for the last couple bytes
written to the buffer.

This change removes buffering from SSTSnapshotStorageFile
as pebble's SST writer already does its own buffering. This
makes Sync() do what it is expected to do.

Release note (bug fix): Fix a bug where a badly timed power outage
or system crash could result in an error upon process restart.

[cockroach-teamcity]

This change is 

[petermattis]

LGTM

Did you check whether this bug existed in 19.2?

[itsbilal]

TFTR!

And yes, I just checked. It seems like snapshots aren't applied through ingestions in 19.2 at all - we seem to be doing ApplyBatchRepr's and then following the batch commit pipeline. All of that stuff seems safe enough.

[itsbilal]

Nevermind, that was 19.1. Checking 19.2 right now.

[itsbilal]

Yep, in 19.2 we just use RocksDB's DBFile->Append() and ->Close(), and the WritableFileWriter definitely seems to be syncing on close, so this bug doesn't exist there.