Add Prepare API

[RichLogan]

Adds a prepare API whereby a sequence number can be submitted to call any appropriate concealment callbacks in order to make the buffer ready for the Enqueue of said sequence number's packet data. This is useful when your concealment is stateful and you need it to be generated before you decode your to-be-enqueued data.

For example:

• Packet 1 has arrived, been decoded and Enqueue'd into the buffer
• Packet 3 just arrived.
• Call Prepare(3) to generate and enqueue missing packet 2
• Now we are in a state where 3 can be decoded and Enqueue'd

[GhostofCookie]

I think this looks good!