Fixed: WebSocket _changes feed wouldn't connect over SSL

It would try to connect over plain HTTP and fail, because I put the
-startTLS: call too early.
Also, I enabled cert hostname verification, which is an important safe-
guard against MITM attacks.

Fixes #332
Fixes #333
Merge remote-tracking branch 'upstream/master'
CBLNestedModels can be expensive to create. Therefore, we changed didSave: to not invalidate the cached property values. Instead, we register to listen for UIApplicationDidReceiveMemoryWarningNotification, and remove all unchanged properties if received.