Add nbodies simulation for statistic analysis.

Only configuration is really basic right now, just for testing the
core.
Add visualiser for nbodies simulation.
Skip clock skew events in nbodies visualisation.
Add a lot more instrumentation to the system stack.

Large numbers of simultaneous connections cause some nodes to fail to
discover others, so trying to dig into the root cause here.
Fix a number of issues that could arise when many nodes join simultaneously.
Don't draw wrap-around lines in nbodies visualisation.
Tweak settings in vnet-8-latent-lossy config to match name.
Tweak physics of nbodies-core.

Old settings had way too much acceleration, which made the visualisation
hard to read, since much divergence could occur simply due to one-way
latency.
Remove dead *exttime support from virtual-network.
Fix off-by-one in CPP check for MSVCRT-like rand().
Add bandwidth report to nbodies log.