icingadb.DB: Retry Schema Checks

During testing, I just encountered a race condition where my Galera
cluster was not yet ready, causing the initial schema check to fail.

```
2024-04-11T08:13:40.401Z        INFO    icingadb        Starting Icinga DB daemon (1.1.1)
2024-04-11T08:13:40.401Z        INFO    icingadb        Connecting to database at 'mysql:3306'
2024-04-11T08:13:40.404Z        FATAL   icingadb        Error 1047 (08S01): WSREP has not yet prepared node for application use
can't check database schema version
github.com/icinga/icingadb/pkg/icingadb.(*DB).CheckSchema
        /go/src/github.com/Icinga/icingadb/pkg/icingadb/db.go:115
main.run
        /go/src/github.com/Icinga/icingadb/cmd/icingadb/main.go:74
main.main
        /go/src/github.com/Icinga/icingadb/cmd/icingadb/main.go:37
runtime.main
        /usr/local/go/src/runtime/proc.go:271
runtime.goexit
        /usr/local/go/src/runtime/asm_amd64.s:1695
exit status 1
```

This change now also retries the initial cluster check.

References #698.

pkg/icingadb/db.go

@@ -110,7 +110,14 @@ func (db *DB) CheckSchema(ctx context.Context) error {
 
 	var version uint16
 
-	err := db.QueryRowxContext(ctx, "SELECT version FROM icingadb_schema ORDER BY id DESC LIMIT 1").Scan(&version)
+	err := retry.WithBackoff(
+		ctx,
+		func(ctx context.Context) error {
+			return db.QueryRowxContext(ctx, "SELECT version FROM icingadb_schema ORDER BY id DESC LIMIT 1").Scan(&version)
+		},
+		shouldRetry,
+		backoff.NewExponentialWithJitter(128*time.Millisecond, 1*time.Minute),
+		db.getDefaultRetrySettings())
 	if err != nil {
 		return errors.Wrap(err, "can't check database schema version")
 	}