Keep a condition's looks in the store, so a look that changes nothing writes no bucket (issue 420)

Every look rewrote the condition's value in the conditions bucket, because the
value carried the count of looks, the last look and the newest evidence: 131 647
writes in four days for twelve open conditions, against ADR 0289's rule that a
bucket is written only when what it describes changed. The looks now live in the
controller's store (migration 0095, condition_looks), the bucket holds what a
condition says and is written only when that changes, and the conditions verb and
the condition events put the looks back.

internal/broker/writers.go

@@ -108,7 +108,8 @@ var WritersTable = []WriterRow{
 	{State: "plans and their tiers", Writer: "controller (lease holder), compare-and-set on the plan's revision",
 		KeptIn: "the controller's store", Others: "read through plans"},
 	{State: "conditions", Writer: "controller", KeptIn: "key-value " + ConditionsBucket + " (and its history, " +
-		ConditionHistoryBucket + ")", Others: "raise or clear only through observations the controller reads",
+		ConditionHistoryBucket + "); how often each was observed, when last and its newest evidence in the " +
+		"controller's store, so a look that changes nothing writes no bucket", Others: "raise or clear only through observations the controller reads",
 		Subjects: append(kvOf(ConditionsBucket), kvOf(ConditionHistoryBucket)...), Writes: isController},
 	{State: "calls and their outcomes", Writer: "controller", KeptIn: "key-value " + CallsBucket,
 		Others: "read by id", Subjects: kvOf(CallsBucket), Writes: isController},