Keep rollouts and the merges they answer in the rollouts bucket (issue 1092)

Once chain_moved records the kind, a plan is a key of the rollouts bucket under its id and the open batch is the same key in state assembling. A save refuses a copy another save moved since (the plan's own Revision, kept in its value, counts saves) and keeps the merges the record holds, so a copy read before a merge moved cannot lay it back. The merges a batch or walk answers are the record's held field, with the columns the table had; a merge no plan answers stays, flagged, in the record that held it last, or in the record kept for those. A merge moved to a plan is written to the receiving record first and then taken from the giving one; one found in both is read from the one it entered last and repaired at the next cut. Plans and merges are read from a view kept by one watch of the bucket. A bus that does not answer, a lease lost and a key moved are now not-now like the store being away, so a merge whose plan could not be written is held and handed over again, never acknowledged. The start moves release_plan and batched_merge into the bucket, one kind, counted against both tables.

internal/chain/records.go

@@ -143,6 +143,22 @@ type Links struct {
 	Changed []Change `json:"changed,omitempty"`
 }
 
+// HeldMerge is a merge a rollout holds: the columns the controller's table of batched merges had, kept in the record of the
+// batch or walk that answers it (novox/hq ADR 0276, issue 1092). A merge no plan answers (Waiting) stays in the record
+// that held it last. Placed is when it entered this record, so one found in two records after a crash between the two
+// writes is read from the one it entered last.
+type HeldMerge struct {
+	Repository string          `json:"repository"`
+	Branch     string          `json:"branch,omitempty"`
+	Commit     string          `json:"commit"`
+	Merged     time.Time       `json:"merged,omitzero"`
+	Heard      time.Time       `json:"heard,omitzero"`
+	Event      json.RawMessage `json:"event,omitempty"`
+	Alone      bool            `json:"alone,omitempty"`
+	Waiting    bool            `json:"waiting,omitempty"`
+	Placed     time.Time       `json:"placed,omitzero"`
+}
+
 // Rollout is the controller's record of one batch's delivery, kept under the batch's id: the commit it fixed per
 // repository, its tiers, the builds it carried and the assignments it changed (ADR 0408 §1). The controller's plan
 // carries more (its modules, its times, its backups) beside these, under the same key.