drop compatibility for versions without the "raw_packets" feature (pre 0.3.x)

[Antoine Martin]

Follow up from #68, this will allow us to simplify the code greatly:

• "_queue_write" becomes a simple inline call to "self._write_queue.put(data)"
• "set_deflate" can be handled outside protocol: just setting the new value asynchronously, removing lots of cruft from protocol written specifically for dealing with threading
• remove the unconditional calls to "self._decompressor.decompress" from the main read thread loop (do it per packet only)
• the code to change the compression level is tricky, and probably racy too... and we can get rid of all that.

[Antoine Martin]

done in r1166

We cannot drop all of it from the read loop though because old clients still send the initial packet in the old format. This can be removed at a later date.

[migration script]

this ticket has been moved to: ​https://github.com/Xpra-org/xpra/issues/122