Improve package loss concealment

Currently, we replace lost packages with silent-packages.

This is the simplest, but also the worst technique, which results in cut off speech when several packages are lost.
A possibly better alternative would be to repeat the last correctly received package. Since the contents of two successive packages are often similar, this will result in much better audio-output when losing packages.
Further improve concealment by adjusting the thresholds (silence, number of copied packages) and replacing silence with comfort noise.

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