Add timeout to search requests with ?wait=true

When using the search endpoints, the current behaviour is to wait for any elasticsearch writes before the search will be performed. This prevents getting outdated data. This will wait until all pending operations to Elasticsearch are complete. However, when there are a lot of operations pending, this could possibly wait forever. There should be a way to provide a timeout after which the search request is sent anyways.

This could be a setting that is configurable in the config and/or a query parameter for each request.