kv: bump kv.range_merge.queue_interval to 5s

Informs cockroachdb#62700.

This commit bumps the default value for the `kv.range_merge.queue_interval`
cluster setting from 1s to 5s. This setting serves as a per-store rate limit on
the number of range merges that will be initiated. We've seen in a few issues
like cockroachdb#62700 that excessive range merge traffic can cause instability in a
cluster. There's very little reason to be aggressive about range merging, as
range merges are rarely needed with any urgency. However, there are good reasons
to be conservative about them.

This change can also be justified as a (late) reaction to the increased max
range size from 64MB to 512MB. A range merge may need to rebalance replicas in a
range, so its cost can be a function of the sizes of ranges. This means that if
range merges are now more expensive, we should be running them less frequently.

pkg/kv/kvserver/merge_queue.go

@@ -47,7 +47,7 @@ const (
 var MergeQueueInterval = settings.RegisterDurationSetting(
 	"kv.range_merge.queue_interval",
 	"how long the merge queue waits between processing replicas",
-	time.Second,
+	5*time.Second,
 	settings.NonNegativeDuration,
 )