Work on failed outbound requests and move type migrators

- DB type migrators are now in separate file, in preparation for full
  custom sql migration statements
- Start work on handling failed outbound requests stored in the db

storage-new/models/ServerMetadata.go

@@ -12,6 +12,7 @@ type ServerMetadata struct {
 	UpdatedAt time.Time
 
 	// ---- Section TLS LetsEncrypt
+
 	LEDomain            string
 	LECertUrl           string
 	LECertStableUrl     string
@@ -21,4 +22,7 @@ type ServerMetadata struct {
 	LECSR               []byte // Encrypted
 	LELastUpdate        sql.NullTime
 	LEUserPrivKey       []byte // Encrypted
+
+	// ---- Section Database
+	LastMigrationVersion uint64
 }