kv: move node status range to system RPC class

[nvanbenschoten]

We currently have two ranges that qualify for the system RPC class, which provides isolation from networking congestion on data-plane RPC connections:

cockroach/pkg/rpc/connection_class.go

Lines 61 to 64 in 967fe5e

	var systemClassKeyPrefixes = []roachpb.RKey{
	roachpb.RKey(keys.Meta1Prefix),
	roachpb.RKey(keys.NodeLivenessPrefix),
	}

This is important in the context of network saturation like that described in #111238.

In a support case, we saw that the range spanning from /System/NodeLivenessMax to /System/tsd (ID 4, created by these static split points) was impacted by traffic on other ranges. This prevented nodes from restarting because a restart writes to a node status key. Had the range been part of the system RPC class, it would have been fine and the raft transport saturation on the data ranges would have been less impactful.

We should move this range to the system RPC class, as it stores a few important keys and should otherwise see little traffic. Doing so will require some changes to systemClassKeyPrefixes, as unlike the previous two ranges, this one is not part of NoSplitSpans, so it can technically split.

Jira issue: CRDB-31831

Epic CRDB-32846