Common changes for all artifacts

Updated dependencies to latest versions (AGP 4.1, OkHttp 4.9, Coroutines 1.3.9, ExoPlayer 2.12.1, etc.)
- See PR #757 for full list of version updates
Revamped Call implementations
- The Call2 type has been removed, the libraries now all use the same Call instead for all APIs
- Call now guarantees callbacks to happen on the main thread
- Coroutine users can now await() a Call easily with a provided extension

stream-chat-android

Add empty state views to channel list view and message list view components
Allow setting custom empty state views
Add loading view to message list view
Allow setting custom loading view
Add load more threshold for MessageListView and streamLoadMoreThreshold attribute
Fix handling of the streamShowReadState attribute on MessageListView
Add streamShowDeliveredState XML attribute to MessageListView
Add "loading more" indicator to the MessageListView
Messages in ChannelController were split in messages - New messages and oldMessages for messages coming from the history.

Fix guest user authentication
Changed API of QuerySort class. You have to specify for what model it is being used.
Rename ChannelController to ChannelClient. Deprecate ChannelController.
Replace ChannelController subscribe related extension functions with corresponding ChannelClient functions
Move ChannelClient extension functions to io.getstream.chat.android.client.channel package

Add GetChannelController use cases which allows to get ChannelController for Channel
Fix not storing channels when run channels fetching after connection recovery.
Fix read state getting stuck in unread state