Create indexing tasks and jobs

[unclecheese]

As a Silverstripe CMS developer, I want to be able to index content in a search provider as it changes so that I don't have to use a separate tool product to manage my searchable content.

• Indexes are created per environment
• Content can be persisted to multiple indexes
• Can reindex with a synchronously (opt-in)
• Can reindex synchronously (on write)
• Can reindex with task
• Parallelisation is encouraged and used
• Similar or better performance than fulltextsearch module (~20,000 documents)