Data race: worker.Sync() and worker.Recommend()

**Gorse version**
Version: unknown-version
API version: v0.2.7
Go version: go1.19.3
Git commit: 762f21f3154f35877fe376ba435c11e7eaaf5edc

**Describe the bug**
A data race occurs when the worker configuration is updated.
[Sync()](https://github.com/gorse-io/gorse/blob/762f21f3154f35877fe376ba435c11e7eaaf5edc/worker/worker.go#L381) write [w.Config](https://github.com/gorse-io/gorse/blob/762f21f3154f35877fe376ba435c11e7eaaf5edc/worker/worker.go#L155), and [Recommend()](https://github.com/gorse-io/gorse/blob/762f21f3154f35877fe376ba435c11e7eaaf5edc/worker/worker.go#L404) read.

Perhaps it is better to sync the configuration only between reranking cycles? Because the sync operation is much faster than the calculation of recommendations, as well as changing the configuration parameters during the ranking process can lead to incorrect result.

**To Reproduce**
[Test](https://github.com/ingosus/gorse/blob/config_update_race/worker/worker_test.go#L685)
go test -v -race github.com/zhenghaoz/gorse/worker -run TestWorker_SyncRecommend

**Expected behavior**
no data race