requeue pending messages onTransportUnavailable

[SimonWoolf]

Looking at https://github.com/ably/ably-java/blob/master/lib/src/main/java/io/ably/lib/transport/ConnectionManager.java, looks like currently if a transport dies, pending (unacked) messages are left in pendingMessages; they should be requeued (removed from pendingMessages and added to queuedMessages), to be resent when a new transport connects or failed when the lib goes into suspended. See http://docs.ably.io/client-lib-development-guide/features/#RTN19a

┆Issue is synchronized with this Jira Task by Unito