Collection-level validator functions

[sanderpick]

I've already been POC-ing this idea, but creating an issue now for tracking. Essentially, allow the user to write a function that validates a update event.

[sanderpick]

The full DB access story is a bit unwieldy... so, breaking it down here:

Collection writes

• NewCollection: Touches thread via CreateThread (Net.ValidateWrite)
• DeleteCollection: Touches thread via DeleteThread (Hub context + Net.ValidateWrite)
• UpdateCollection: Does not touch thread, but will when collection info is persisted to thread records (Hub context + Net.ValidateWrite)

Collection reads

• GetCollection: Does not and won't ever touch thread (Hub context + Net.ValidateRead)
• ListCollections: Does not and won't ever touch thread (Hub context + Net.ValidateRead)

Data reads

• Find: Does not and won't ever touch thread (ReadFilter + Net.ValidateRead)
• FindByID: Does not and won't ever touch thread (ReadFilter + Net.ValidateRead)
• Has: Does not and won't ever touch thread (ReadFilter + Net.ValidateRead)
• Listen: Does not and won't ever touch thread (ReadFilter + Net.ValidateRead)

Data writes

• Create: Touches thread via CreateRecord (WriteValidator + Net.ValidateWrite)
• Save: Touches thread via CreateRecord (WriteValidator + Net.ValidateWrite)
• Delete: Touches thread via CreateRecord (WriteValidator + Net.ValidateWrite)