Proper and fast encryption support

As encryption is a very important part of Peergroup I decided to create a new release only for this change.

Previously the key derivation algorithm was invoked in every thread that handles a sent/received data chunk. In fact we only need to derive the encryption key from the channel password once.

This is fixed in this release so encrypted file transfer is no longer slower than unencrypted transfer (at least if you have something more high-performance than an raspberry pi ^^).

Beta with encryption support

In this release the auto-generated sample config file contains extensive comments for each field. Additionaly, the user can enable/disable data transfer encryption in the config file. Last but not least, the port used for P2P data traffic is chosen randomly in the range 49152-65535 if it is not explicitly set in the config.

Now that I fixed some severe bugs (see beta0) I find this version pretty stable and worth a try!

Beta (fast, without encryption)

I fixed all known severe bugs in this release. However, I disabled data transfer encryption, because it has a larger impact on performance than expected the way it is implemented currently.