feat(store): drift state, move candidates and service merge (#10)

Adds services.awaiting_inventory, Service.Drifted/RunsRef/RunningRef, AcceptRunning, MoveCandidates, MergeSources and MergeService, and maps a job for a deleted service to ErrNotFound.

internal/master/model/job.go

@@ -44,6 +44,27 @@ const (
 	ReasonNoTag     = "follows no tag"
 	ReasonRemoved   = "service no longer exists"
 	ReasonBuilt     = "built from source (build:)"
+
+	ReasonDrifted    = "drifted: restore or accept the running version first"
+	ReasonNotDrifted = "runs its desired version"
+)
+
+// MergeRefusedError says why two services cannot be merged. Reason is shown
+// in the UI as is.
+type MergeRefusedError struct {
+	Reason string
+}
+
+func (e *MergeRefusedError) Error() string { return "cannot merge: " + e.Reason }
+
+// Reasons two services cannot be merged. ReasonSelf is reused for the
+// updater itself.
+const (
+	ReasonMergeSame      = "a service cannot be merged into itself"
+	ReasonMergeLiveOld   = "the old service still exists"
+	ReasonMergeGoneNew   = "the new service no longer exists"
+	ReasonMergeKind      = "only services of the same kind can be merged"
+	ReasonMergeContainer = "plain containers cannot be merged"
 )
 
 // JobStatus is the state of an update job.