kvserver: default kv.range_split.load_sample_reset_duration to 30m

See #144407 - the split reservoir can sometimes contain "old state" when the
load based splitter is persistently engaged. When the splitter can make a
split, it typically is ready to do so in a minute or two, though the range also
needs to be picked up by the replica scanner, which is only guaranteed to come
around every ~10 minutes. So resetting it every "tens of minutes" is not
disruptive but can help ranges getting stuck in a state where they deserve a
split but will not perform one.

Release note (ops change): the kv.range_split.load_sample_reset_duration now
defaults to 30m. This should improve load-based splitting in rare edge cases.

Author: tbg

pkg/kv/kvserver/replica_split_load.go

@@ -74,7 +74,7 @@ var SplitSampleResetDuration = settings.RegisterDurationSetting(
 	"the duration after which the load based split sampler will reset its state, "+
 		"regardless of any split suggestions made, when zero, the sampler will "+
 		"never reset",
-	0, /* disabled */
+	30*time.Minute, // a multiple of the default scan interval of 10m
 	settings.DurationWithMinimumOrZeroDisable(10*time.Second),
 )