Made CBLWebSocketChangeTracker work correctly with CBLAuthorizer

Also found that it didn't work correctly if usePOST property wasn't set,
so I fixed that too.
Fixes #400
Skip FMDB thread-safety check when running on dispatch queues.

Fixes #376
Better handling of a model's document being deleted

* Clear all properties
* Clear needsSave flag, remove from db's docs-to-be-saved set
* Detach from document even if the model didn't initiate the save
* Don't call didLoadFromDocument
Fixes #363
Add JSONObjectWithDate:timeZone: to get a date with tz-offset.