Fix obsolete docs deletion

Related to dbnomics-fetchers/management#761 (closed)

Important points:

• commit posted documents so that they can be queried from the delete_obsolete_documents function
• use datetime values for indexed_at, so that solrq escapes them correctly
• hack: build solr queries half with solrq, half manually, because I could not make work the query with parentheses generated by solrq
• Do not process provider if there are no datasets to process: avoid updating the provider document for a no-op