Packet reordering and packet loss handling

[nisay759]

These changes aim to circumvent some problems that may occur when packets don't arrive in the right order (due to network latencies or short sleep times):

• The first commit is for reordering the intermediate packets upon receival of the last ("done") packet.
• The second commit handles the case when the last ("done") packet arrives while some intermediate packets are still on their way to the server: In this case, the last packet's id is used to deduce the number of packets composing the entire message, and the retrieve_file() function can be called when this number is met.

[PaulSec]

Hey @nisay759, thanks a lot for your PR. However, if you want to contribute to this project, you should add your PR to my personal repository (@PaulSec) since I don't maintain the SensePost one anymore.

All the best!

[nisay759]

Alright ! I'll do this right away.