Automatically calibrate server entropy source to determine load limits

[steveatinfincia]

Server should test it's local hardware entropy source when it starts before accepting clients to see how fast it can provide samples to client machines.

We can then use that information to set either:

• A static client limit
• A QoS type guarantee of entropy KB/s per client

Initial implementation of an RNG performance test method committed at 6f21e69

Will still need to further refine the test method for accuracy, and then write some code to actually do something with the results.

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