File creation

[Alystrasz]

Currently, on the receiving end, file creation is done in a synchronous fashion: all file chunks are stored in memory upon reception, and exported to file in order. This prevents transfering files bigger than receiver device's memory size, and does not allow failure recovery in case file exchange goes wrong.

1. Receiver end should acknowledge file chunks only after they have been stored somewhere safe.
2. Moreover, instead of saving all chunks in memory, we should write them directly in destination file at correct position (use a RandomAccessFile instance to allow this).