v0.6.0: Absolute PreImage offsets, greatly improved Flash API, transaction fees

Notable changes since the last release:

• Validator now authenticate themselves upon connecting to one another;
• The random seed algorithm has been changed to be simpler;
• The node hashing algorithm has been changed to be stable at a given height/state;
• Many bugs in the shutdown procedure have been fixed, and things should never hang now;
• The consensus protocol now supports a minimum fee;
• Default development compiler is now LDC 1.26 (LDC 1.25 works for building, but not in unittest mode);
• Windows builds have been disabled due to limitations in the CI memory capacity;
• PreImages now uses absolute index (height) instead of relative index (relative to enrollment);
• The Flash layer now supports managing multiple keys from the same node;
• It is now possible to list flash channels through the API;
• The node now exposes 4 config parameters to control and limit transactions propagation;
• Flash: Settlement is now always published after timeout is reached;
• Talos, the setup interface, is now integrated in Agora and can be started with --initialize=address_to_bind;
• The API now allows to get detailes informations about a list of Flash channels;
• The Flash control API can now be configured to bind to a specific address/port;
• Rules for nomination has been improved to take fees into account and better handle time differences;
• Loggers can now be reconfigured dynamically through the admin interface;
• Nodes will now prioritize their own Enrollment instead of waiting to hear back from other nodes;
• DataPayload now uses Base64 for the REST API;
• DataPayload.data was renamed to bytes;
• SCP is now built with clang++ on all platforms;
• The random seed check no longer mistakenly uses the node state over the block state;