Fix: Queued job for indexing is unreliable

[sig-peggy]

The Firesphere\SolrSearch\Jobs\SolrIndexJob is not reliable at indexing a full site. Some issues noted:

• When the indexing is divided into groups it doesn't always create as many jobs as there are groups
• There is no way to use the clear=true flag that's used in the SolrIndexTask to empty the index before re-indexing which is frequently required, especially if there has been any change to the configuration
• The Job may not find the same or as many objects to index as the SolrIndexTask

Expected behaviour
A queued job should be available to fully reindex a site, including clearing anything already in the index first. To accommodate large sites, all indexed objects should be divided into groups that each have their own separate job so that if one job in the middle fails the others around it can continue.
Unclear if it's worth using the Firesphere\SolrSearch\Jobs\SolrIndexJob as a base or if it might be easier to start over from scratch.