Fixes:
• Sometimes Service Nodes aren't reliable. Session now correctly retries with a different Service Node if a message failed to send.
• Fixed various multithreading bugs that could cause poor performance or undefined behavior.
• Fixed various closed group bugs.
Improvements:
• Session now offers the choice between using APNs and background polling (previously the default) for push notifications. APNs provides much more reliable and immediate push notifications.
• Heavily refactored the service node communication layer for greatly improved reliability.
• Various minor UI improvements.