Fixes before Beta

[mgratch]

• add Multiselect UI on the view to allow the admin to determine which columns are sortable and whether they are ascending or descending.
• Only show visible columns in Multiselect UI drop down
• Ensure plugin only works with DataTable Views
• Make sure admin scripts are initialized when datatable view is selected for the first time.
• Ensure admin scripts only fire on edit.php page while adding/editing a View
• Add failed background processing checks (health checks)
• Convert MySQL PDO code to MySQLi code ... i.e. the proper WordPress way
• Check for background processing library whether plugin or dependency
• Remove index table when Views are trashed
• Add uninstall function
• Add remove tables on uninstall setting
• Enqueue creating index tables for existing DT Views on activation
• Make multi-select sort field UI sortable with drag and drop + touch
• General Code Clean Up
• Dynamically identify how many rows can be inserted at a time without causing strain on the server (currently it is processing 250 at a time and inserting 1 row)
• Add notice to GV edit.php table which tables are indexed

[zackkatz]

• Namespace tables; instead of queue and failed_jobs, perhaps gv_entry_index_queue gv_entry_index_failed_jobs
• Log activity using gravityview_log_error and gravityview_log_debug hooks, or something else like WP-Logging