Longer bulk load timeout

Use 30 second timeout when doing bulk indexing.

The new bulk_load script is now being used by whitehall to do a full re-index, but we're seeing intermittent timeout errors connecting to elastic search:

https://deploy.production.alphagov.co.uk/job/Whitehall%20Reindex%20Elasticsearch/8/console

Increasing the timeout should hopefully allow this job to complete successfully and reliably.

@bradleywright @fatbusinessman