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/imageref/imageref.go

@@ -2,7 +2,10 @@
 // and digest.
 package imageref
 
-import "strings"
+import (
+	"regexp"
+	"strings"
+)
 
 // Ref is an image reference split into its parts. Tag and Digest are empty
 // when the reference has none.
@@ -41,3 +44,8 @@ func (r Ref) String() string {
 	}
 	return s
 }
+
+var digestPattern = regexp.MustCompile(`^sha256:[0-9a-f]{64}$`)
+
+// IsDigest reports whether d is a well-formed sha256 digest.
+func IsDigest(d string) bool { return digestPattern.MatchString(d) }