release-20.1: raftentry: fix accidental no-op in truncateFrom

[tbg]

Backport 3/3 commits from #63302.

/cc @cockroachdb/release

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When entries are appended to the raft log, they are also added to the
raft entry cache. Appending indexes, say, 100, 101, 102 to the raft
log has the (absolutely crucial for correctness) effect of removing any
higher indexes in the raft log ( 103 and up) and for correctness, the
raft entry cache must follow the same semantics, so that it is always
a coherent subset of the raft log.

It turns out it didn't do that in one edge case, namely when appending
a slice of entries that didn't fit into the cache, but whose first
entry preceded the currently cached entries. For a concrete example,
imagine the cache contains

[101, 102, 103, 104]

and we are attempting to add a slice of large entries at indexes [50, 51, 52] which can not fit the cache at all (i.e. their aggregate size
exceeds that of the cache).

What should be the result of this addition is an empty cache. This
is because the true raft log now ends at index 52. In reality though,
the cache accidentally turned this operation into a no-op, as it was
using an iterator that would be invalid when positioned to an index
not currently cached (in this example, index 50, whereas the first
cached index would be 101).

It took us a while to identify this bug, as it manifested itself to us
only through a very rare replica inconsistency failure (#61990) of the
hotspotsplits/nodes=4 roachtest (which was only caught thanks to
#36241) that would then take days to reproduce. When we finally
managed to get a reproduction that contain the entire history of disk
writes for the cluster, we found that the inconsistency had occured
due to entries from a past term being replicated to the divergent
follower. Concretely, where the leader had something like

idx	101	102	103	104	105	106	107	108
term	9	9	9	9	9	9	9	9

the divergent follower would have this:

idx	101	102	103	104	105	106	107	108
term	9	9	7	7	7	9	9	9

This made it likely that the issue was connected to a raft leadership
change in which the "old" leader for term 7 had proposed entries for
indexes up to and including at least index 105, which the term 8 and
9 leaders had then discarded and replaced by entries for term 9. Of
course, these entries should consequently never have been committed
and in particular, the term can never regress in the raft log, even
for uncommitted entries, at least when Raft is working correctly.

From the WAL writes, we were able to ascertain that the leader (who
itself had the correct writes in the log) had in fact replicated the
incorrect (term 7) entries to the follower. This meant that that we
were either looking at pebble flat-out serving stale reads (highly
unlikely) or a bug in the raft entry cache. Being unable to find
issues through direct inspection, we added invariants and tweaked the
test to exacerbate what we considered the triggering conditions
(relatively frequent term changes).

A crucial assertion that allowed identifying this bug was adding
invariant checks (which will be re-added in a future commit) that
verified that the truncation had indeed removed subsequent entries.
This highlighted that we were hitting this bug in at least ~5% of
hotspotsplits runs, but possibly more frequently (the assertion did not
catch all instances of the bug). hotspotsplits brings the crucial
ingredients here: it's doing concentrated 256KiB inserts which puts the
raft group into mild overload; many entries pile up and get processed in
aggregate, which leads to constantly taking the path in which
truncations are erroneously ignored. However, most of the time omitting
the eviction does not yield replica inconsistency. For this to happen,
the erroneously cached entries need to be picked up by raft for sending
to a follower; they need to be the "incorrect" (which necessitates a
discarded log and term change at the right moment), and finally they
must be in log positions at which the correct entries actually had an
effect (which is to be fair mostly the case).

The history of this bug is actually interesting. The code that has
the bug was introduced in #36360. However, that PR fixed a much more
egregious version of this bug - prior to that PR, the raft entry
cache simply didn't evict entries at higher positions at all, i.e.
it exhibited the bug in many more scenarios, and this consequently
lead to more frequent inconsistencies (as referenced in the issue).
So we went from bad, moderately rare bug to bad, very very rare bug
and, in this PR, hopefully, to no bug.

A rough calculation suggests that we spent around 800.000 minutes of
n2-standard-4 instances minutes on reproductions related to this bug
in this instance alone, which I think boils down to around $39.000. I
like to think we got off cheap here.

Closes #61990.

Release note (bug fix): A rare issue that could cause replica divergence
was fixed. These issues would be reported by the replica consistency
checker, typically within 24 hours of occurrence, which would cause
nodes to terminate.

[cockroach-teamcity]

This change is