* core: import and export of chat archive
* export chat archive
* import archive, support starting chat after it is stopped
* test for maintenance mode
* test/fix archive with files
* prevent starting chat after chat database was deleted or imported
* update simplexmq
* core: batch contact disconnections and re-subscriptions to one event per server
* update simplexmq
* update query to use temp table
* remove old code
* ios: get device token for push notifications
* ios: receive messages when background notification is received
* add notifications API, update simplexmq
* chat API to register and verify notification token
* update AppDelegate to recognize different notification types, update simplexmq
* core: api to enable periodic background notifications
* update simplexmq
* chat API to delete device notification token
* use base64url encoding in verification code
* update simplexmq for notifications