Files in multi-value file fields are removed from the index upon re-indexing if the field has multiple files.

To reproduce:
1. Create node with multiple files in a multi-value file field.
2. Allow Solr to index those files.
3. Trigger re-indexing by clicking "Delete the attachments from the index" in apachesolr_attachments settings page.
4. All but one file in the field are now gone from index, because re-indexing only gets one file.

This is due to the use of EntityFieldQuery to get the list of files to index in _apachesolr_attachments_get_all_files(). EntityFieldQuery, by design, will only return a single stub record per entity. This means that each entity that contains files has one result row, and each row has a single file ID which is identified for indexing.