Enhancement Request: Option to set the order of offline messages and more

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- [x] Bug exists Release Version 1.1.0 ( Master Branch)
- [x] Bug exists in Snapshot Version 1.1.1-SNAPSHOT (Develop Branch)

If this is a bug regarding the Android Service, please raise the bug here instead: https://github.com/eclipse/paho.mqtt.android/issues/new

Presently delivery of offline messages takes place in the order it was produced and only single buffer of max size 5000 msg is supported. In situations where data rate is high and network is down for prolonged duration, there is a possibility of loss of data due to small size and number of offline buffer.
Also, after connectivity is restored it may take quite a long time to clear the backlog. In application where latest data is of higher relevance, the application will have to wait many more minutes, even after connectivity is restored, before it starts seeing useful data.

I would like to have following enhancements to support my use case:

- Possibility to set the order of offline message to be sent in a buffer(FIFO/LIFO)
- Possibility to have N number of active offline buffers with size 5000. Max number is set by publisher but buffer is created if required.
- Possibility for subscriber to archive/delete buffers if clearing them does not make sense (based on time elapsed, rules, brute force)
- Possibility to set order of offline buffers to be chosen for clearing (FIFO/LIFO)

Thanks