Implement asynchronous communication

[hiddemoll]

PR #1119 made the management unit start in parallel with clock control, but it only continued to UGN capture after clock control was finished. We want these 2 CPUs to be completely independent.

The idea is that we center the elastic buffers and capture the UGNs. From that point on, the links can be used for arbitrary communication. We'll keep on moving the buffers to their midpoints to prevent overflows (given that clock control still might be correcting clocks), so we'll use eb_changes to calculate the final UGN once we determine the whole network is up and therefore stop modifying the buffer occupancy.