kvserver: add assertions for invariants around up liveness records

Now that we have #53842, we maintain the invariant that there always
exists a liveness record for any given node. We can now simplify our
handling of liveness records internally: where previously we had code to
handle the possibility of empty liveness records (we created a new one
on the fly), we change them to assertions that verify that empty
liveness records are no longer flying around in the system.

When retrieving the liveness record from our in-memory cache,
it was possible for us to not find anything due to gossip delays.
Instead of simply giving up then, now we can read the records directly
from KV (and evebtually update our caches to store this newly read
record). This PR introduces this mechanism through usage of
`getLivenessRecordFromKV`. We should note that the existing cache
structure within NodeLiveness is a look-aside cache, and that's not
changed. It would further simplify things if it was a look-through cache
where the update happened while fetching any record and failing to find
it, but we defer that to future work. A TODO outlining this will be
introduced in a future commit.

A note for ease of review: one structural change introduced in this diff
is breaking down `ErrNoLivenessRecord` into `ErrMissingLivenessRecord` and
`errLivenessRecordCacheMiss`. The former will be used in a future commit
to generate better hints for users (it'll only ever surface when
attempting to decommission/recommission non-existent nodes). The latter
is used to represent cache misses. This too will be improved in a future
commit, where instead of returning a specific error on cache access,
we'll return a boolean instead.

---

We don't intend to backport this to 20.2 due to the hazard described in
\#54216. We want this PR to bake on master and (possibly) trip up the
assertions added above if we've missed anything. They're the only ones
checking for the invariant we've introduced around liveness records.
That invariant will be depended on for long running migrations, so
better to shake things out early.

Release note: None

pkg/kv/kvserver/helpers_test.go

@@ -514,12 +514,9 @@ func (nl *NodeLiveness) SetDrainingInternal(
 }
 
 func (nl *NodeLiveness) SetDecommissioningInternal(
-	ctx context.Context,
-	nodeID roachpb.NodeID,
-	oldLivenessRec LivenessRecord,
-	targetStatus kvserverpb.MembershipStatus,
+	ctx context.Context, oldLivenessRec LivenessRecord, targetStatus kvserverpb.MembershipStatus,
 ) (changeCommitted bool, err error) {
-	return nl.setMembershipStatusInternal(ctx, nodeID, oldLivenessRec, targetStatus)
+	return nl.setMembershipStatusInternal(ctx, oldLivenessRec, targetStatus)
 }
 
 // GetCircuitBreaker returns the circuit breaker controlling